And how to avoid them! As a note, even veteran DM's can make some of these errors so don't forget to be careful!
A tabletop can be a crazy place, but to avoid a rebellious player uprising keep these tips in mind.
I think it's also worth mentioning that I know articles like this and boatloads of Dungeon Master advice are all over the blogosphere, so today I'm going to try and approach some problematic areas you may not immediately think of, but in the end always seem to crop up.
As a final note, these are numbered but not in any particular order.
1. Names Don't Matter.
Why this is wrong: Names are small but important building blocks which maintain the consistency of the world your characters are playing in. Obviously, names of characters, NPCs, and locations add up to a large amount of rather random information, but keeping track of these names and encouraging your players to do the same will keep the reality of the world alive.
Solution: Take notes. This is generally a good idea, but especially useful for names. Have a whole page or two devoted to names that you can refer to. I suggest referring to your players by their character's names, even in combat, and encouraging your players to do the same. Small name-plate note cards can be used initially to help this along, or "Hello. My Name is _____" stickers.
Also, in terms of NPCs, nothing makes NPCs seem more insignificant then the DM having to take five minutes just to create a name for them. Use a name generator or your own creativity to come up with a bank of 10 male and 10 female names that you can have on tap for just such scenarios. If your players have their characters walk into a random shop and you have the shopkeeper ready with a name, your players will suddenly give the encounter much more attention, assuming that it's important. Trust me, it works!
2. Not Making Expectations Clear.
Why this is wrong: Everyone plays tabletop games differently. Different DMs work out different social contracts with their players (refer to the great blog Gnome Stew for some additional information on social contracts) and expect different things from them. When a game starts, both the Dungeon Master and the players are expecting different things out of the game.
As the Dungeon Master you cannot simply expect your player's expectations to match up with yours.
Solution: Communication. Tell your players what you expect out of them and out of the game. This can be anything ranging from out-of-character chatter, to who's bringing the snacks, to house rules, to whether or not drinks go on coasters. You cannot be mad at your players for, as an example, saying "I only have 5 hit points left" unless you've told the group that they shouldn't tell other people that information.
Your players can't follow your expectations unless you actually share them. Communication is key.
3. Downplaying Character Death.
Why this is wrong: Character death in any campaign, regardless of system, is a big deal. Even though your NPCs die by the truckload, when a player character dies, HANDLE WITH CARE. Such moments can be emotional ones for any player, especially if they really enjoyed playing the character. NEVER brush off a character death like is nothing. It should be an important moment in your campaign, so do not neglect to treat it like one.
Solution: There are many ways to deal with a character death appropriately. First and foremost, the best method to handle them is by talking about the possibility of character death before the game even starts. Explain that death is a possibility, and what the options are for players should their character die - such as taking over an NPC, building a new character, waiting for resurrection, etc.
The second step you should take with character death is making a big deal out of it during the game. Whenever a character dies in one of my games, I change the music immediately to my death theme (which starts off sad but then powers up with these crazy drums) and take a good minute to describe dramatically how the character dies. I then carry on the fight, with their allies fueled by the warlike music as they avenge their fallen comrade, giving everyone a small boost (+1 or +2) to rolls for the remainder of the fight.
The third step in facing character death is to NOT make the player sit out. Even if a character isn't available for them to play, give them some personal attention in a side quest - for example in a high fantasy setting as they meet their god or goddess. Give them access to something special because they died - maybe a divine meeting, or a peek into a cut scene that their ghost sees. This will make them feel less like they died in vain.
4. All Good Villains Vanish.
Why this is wrong: One of the most annoying things that can happen in a campaign is having the villain simply vanish or teleport away just when they're nearing death. Now, before I go any further, there are many reasons why you might do this that are completely legitimate. If it is built into this character to have such abilities, it makes sense that they would retreat.
What you want to avoid however is the idea of "Wow, they weren't supposed to do that much damage to him. He's supposed to be a recurring villain. He was supposed to escape. Ummm..." and then make him teleport - even though the enemy had no capability to do so - just so you can hold onto a villain for the future.
Solution: If you throw a villain at the PCs who is "supposed to get away", make sure they have treasure that allows them to do so in their inventory. If they've been rubbing a ring of teleportation for the last seven battles, that ring better damn be on that body when the players finally drop him. Along these lines however, don't throw a ring of teleportation onto a villain without it being accounted for cost-wise. Such items are expensive, and should not just be freely given to annoying CR 2 villains willy nilly.
There are many ways to solve this issue, truth be told. The bottom line is that you should never throw an enemy at your players where it's destruction would ruin your story. When PCs see a foe, they're going to try their hardest to kill it, and may pull out stops you weren't prepared for. Be prepared for the possibility that any enemy you throw at them might die, and don't toss in countermeasures at the last minute because then you weren't really giving your PCs a fair chance to begin with, which trust me - they'll catch on to.
5. NPCs Can Be The Heroes Too.
Why this is wrong: Well, it's not completely wrong. NPCs can be great heroes. In fact, any fantasy world worth its salt is full of heroes which drive its legends and tales. What is wrong however, is the fact that NPCs could be heroes in your story. In your story, the player characters are the heroes. Sending uber-powerful NPCs in to save the day and your PCs, or having them accompany your players on an adventure only to mercilessly show them up will just leave your players wondering why they're even there.
Solution: If you have NPCs join your heroes - whether as the cavalry or accompaniment on a journey - ensure that they never take the limelight away from the PCs since they are the true heroes of the story. If your PCs feel under appreciated or not needed, it will make them start questioning why they ever went on the adventure in the first place while there were clearly much more able-bodied NPCs just kicking around. Awesome NPCs can exist without overshadowing the true and rightful heroes of the story - your players.
6. Relying On Initial Story Hooks.
Why this is wrong: Every adventure begins with a story hook. Sometimes an individual hook for each character is needed to pull together the party and send them on their way. Sometimes, PCs will decide to stick together and form an adventuring party. When this happens, fantastic. You've lucked out!
However, more often then not, especially in a party with strong-headed characters, it won't be long after the campaign starts (especially after their first quest ends) that your players might start wondering why they're still sticking around with these people.
Solution: Bottom line? You can't rely on the initial story hooks to keep your players together. Obviously your players should understand that a party needs to exist for the game to work, but it's your job to give the characters a reason to stick together.
Give your players a reason to have their characters stick together. There are hundreds of ways to do this, whether through dreams/visions, a bounty on their heads, or any other such method. The trick is to either impress the fact that either it will be in their best interests to stick together, or it would be a particularly bad decision to split apart.
7. Critiquing Player Roleplaying.
Why this is REALLY REALLY wrong: You should never tell a player how to play their character. Why is this? As the Dungeon Master you control pretty much everything, from the world to the NPCs to the storyline and onward. The only real control a player has is over their characters. If you don't like the way a player is playing their character you have to remember that players put a lot of time and energy into developing their character's personality. It can be severely insulting to a player to critique their roleplaying style.
Solution: If a character is severely interrupting gameplay, talk to the player after the game to find out what's driving their character and what their character's goals are. If a player is having their character act in a rebellious way, it may be because they don't feel like they fit in with the rest of the party, or dislike other party members. If this is the case, bring encounters into the game where teamwork is essential, or where you think the players can learn a little bit about where the particular character is coming from. In the end, the key is to make sure that everyone is having fun. If a player is not having fun playing their character in a particular party encourage them to bring in and try out a new character.
8. Speeding Past In-Character Discussion.
Why this is wrong: When going into any session, a DM has an idea about how much they want to accomplish in that session. Ending on a good note is important after all, but sometimes this can go too far. Putting a time-limit on character interaction, or forcing players to come up with a solution or plan quickly can actually condition your players to act quickly without thinking, and to steer clear of roleplaying and character development.
Solution: Despite how quickly you may want to get from point A to point B, you shouldn't stand in the way of character interaction and careful planning. You shouldn't feel any need to push your players along to the next encounter if they're not ready to move forward. Sit back, put your feet up, and wait for your players to be ready. If they feel rushed and then the next encounter goes poorly, rather then learning from their mistakes they'll wind up blaming you. Never rush character interaction. Enjoy it.
9. Bragging To The Players.
Why this is wrong: Often in any tabletop game, your players are faced with choices. Who they trust, who they capture, which door they go through, how they siege the castle, which wire they cut to disarm the bomb... The life of a DM is hard because you never know ahead of time what choice the PCs will make, so you prepare for each outcome, putting thought into each option.
It's important however, to keep such preparation secret. Why? If the PCs pick Door A, and you brag later about all the treasure they would have gotten through Door B, regardless of whether or not they tried to pry the information out of you, if the missed the better option they're going to be upset.
Solution: If you never brag about what the PCs could have or should have done, then they will - for the most part - never really regret the decisions they made. Believe it or not, the actions most parties make - even inane actions from where you're sitting - are reasonable in their minds given the information that they have.
Keep in mind, they can't always see the whole picture like you can, so they're usually making the best guesses possible, and will usually think "With the information we had, that was the best decision we could have made." When you start telling them otherwise, they feel idiotic and get upset.
So, quite simply, don't brag about what could have happened to your players. Keep what was behind Door B a secret, whether good or bad. Your players may hate that they'll never know, but at least they'll think they were doing what they were supposed to be doing and be confident in their decisions.
10. Excluding A Player.
Why this is wrong: Parties split up and go down opposite tunnels. Party members stay in the tavern or hospital to be nursed back to health or to nurse a cold drink. Thieves strike out on nightly adventures. All of this is fairly standard, but a big mistake a Dungeon Master can make is leaving a player out during a session.
Whether a character stays behind because they don't think the party's being intelligent, or stands outside the cave on guard duty, leaving a player high and dry for an entire adventure just because they didn't follow the adventure's plot precisely can lead to players feeling left out and punished by the Dungeon Master.
Recently in a White Wolf campaign, because my character thought the party was being foolish and going off to their deaths, I stayed at the safe house to wait through the night. Because of that decision, I was left out of the game for three hours. I waited tirelessly with a smile on my face and interest in the game for the Storyteller to throw even the smallest bit of attention my way, but wound up just being bored out of my mind.
Solution: Never let a player sit out. Obviously, going back to #7 you shouldn't force a character to come along on a quest, but you should never let a player feel left out. Whether it's a five minute side-quest, letting them control an NPC, or slightly reworking the story hook to encourage them to come along, anything will work really. If a player is ever sitting for 30 minutes with nothing to do however, you're doing something wrong.
Obviously when a player goes against the grain it can be hard for a Dungeon Master to multi-task or to think up an appropriate side-quest on the spot, but attention to every player is necessary. Give everyone at the table a ten minute break if you need to so you can collect your thoughts and plan. Anything's better then sitting out of a game for three hours, trust me.