probably noticing you did a blunder after you did a move
What's the most annoying thing in chess?
annoying is that most of the opponents wait for you to blunder instead of beating you with good play



Let's see...
1) Making a move and then instantly (and I mean instantly) noticing a mating pattern that I missed. I had 2 of these relatively recently. Ironically, I still ended up winning but I was kicking myself.
2) Waiting for the time to run out in those won positions where the other person just refuses to move or resign.
3) The fact that I have no natural talent for the game whatsoever ... ya ... that's pretty annoying.

having to play a small kid in a tournament who hasn't learned any manners whatsoever.

Having a won chess in hand where opponent has only the king left, but ending up doing stalemate draw.

that is why Fischer invented chess960 ✌️
For me, the most annoying thing about chess is that from time to time I will meet someone who will manage to have the advantage against me during the whole entire game, causing me to feel that there is nothing that I can do to win the game. After that I helplessly see the other player just mentally destroy me on the chess board.
For me, it's when you do a computer analysis on a game and you discover that your opponent's 'best' move turns out to be a blunder that you just weren't able to punish them for.
Anything worse than that?