The top two things I believe a DM should never do are the following
1. Take control of the players characters, as a DM yeah you have all the power in the world literally... you created it. Just don't tell a player his character fought a cr 20 creature and died, when the player wasn't even allowed to have any say in it whatsoever.
2. Rule number two don't ever put your players in a situation that basically has no chance of survival, ESPECIALLY if they weren't doing anything stupid to deserve it. Like really?
Do you guys agree with me or what?
Also list some other things you think DMs should NEVER do.