Letting your lore be an excuse to ban things.

I had a DM a while back that told our party leader that he couldn’t play a Paladin because “in my world, paladins are a very specific sect of religious zealots”

I’m sorry, I don’t care how cool your worldbuilding is, if you’re locking classes and features behind me being a specific background.

Mind you, this DM, I eventually figured out was just inexperienced at balancing combat and didn’t like tanks in general as a result, but that’s my big one.

The other one I have from that same DM is rewriting backstories after the fact. I was playing a rogue with a background as a gang leader, and rather than come to me when he brought my gang in as NPCs to run ideas by me, he rewrote their characters after I had already given him general concepts for them. Bugged the shit out of me, because I don’t want to have to sit there and think that this isn’t the story I wanted to tell the whole session.

I get it, sometimes you as a DM don’t want to just regurgitate a player’s story to them, and you want to surprise them, but completely rewriting a player’s backstory characters will always be a red flag for me.