Number One
Not managing your player's expectations.
If they expect to come to the game session and have an adventure about an evil necromancer, and they do, they won't be upset. If they come to the game expecting a game session about being pirates, and instead there is a game session about an evil necromancer, they will be upset.
If one player creates a character who is a pirate, and another creates a character who is an evil necromancer hunter, it doesn't matter what adventure you create. Someone will be upset.
So TELL THEM UP FRONT what to expect! And if they hate it, work it out amongst yourselves like equals.
Number Two
Thinking "I'm the DM, so I'm always right and what I want to do is the most important."
No, you're not. You're just the guy at the table with the power to make everyone else obey. If you abuse that power, expect that your friends will make you regret it once you take off your DM hat and you're all equals again. With the great power of being a DM comes the great responsibility not to be a jerk about it. That's not just the responsibility to make good rulings as a DM, its the responsibility to do so in a way that doesn't make you a jerk.
Not managing your player's expectations.
If they expect to come to the game session and have an adventure about an evil necromancer, and they do, they won't be upset. If they come to the game expecting a game session about being pirates, and instead there is a game session about an evil necromancer, they will be upset.
If one player creates a character who is a pirate, and another creates a character who is an evil necromancer hunter, it doesn't matter what adventure you create. Someone will be upset.
So TELL THEM UP FRONT what to expect! And if they hate it, work it out amongst yourselves like equals.
Number Two
Thinking "I'm the DM, so I'm always right and what I want to do is the most important."
No, you're not. You're just the guy at the table with the power to make everyone else obey. If you abuse that power, expect that your friends will make you regret it once you take off your DM hat and you're all equals again. With the great power of being a DM comes the great responsibility not to be a jerk about it. That's not just the responsibility to make good rulings as a DM, its the responsibility to do so in a way that doesn't make you a jerk.