For me, it's messing up/ doing something socially unacceptable or hurting someones feelings without noticing. In my life I've done this many times, and the times it happened the worst were when I was having a lot of fun, and had no idea I was doing something wrong. I'm the king of miscommunication.
I had a long history of this, especially as a kid, and because of it for a long time whenever I was having a lot of fun in a social situation I'd suddenly remember past times when I messed up, and then I'd go down in a self-doubt spiral and it'd kill my fun and start acting weird.
The fear that I'm not enough for the people around me. That I'm going to constantly fail the people who need me the most