I'll start!
About 20 years ago, I was at a charity ball which my whole company was attending. It was really, really dull.
I ducked out of the "entertainment" which consisted of unfunny comedians, and singers/bands who clearly hadn't had an adequate sound check, as depsite them being good, could barely be heard over the music. I took myself off to the bar, took my drink outside and just sat there looking at the view (Central London, Thames). A middle aged man, popped out beside me and said something along the lines of "having to take time out to calm yourself from the excitement" totally thought he was being sarcastic, so responded in a really sarcastic tone, something along the lines of "should have put it out to a panel to decide on this entertainment, it's dire!"
He looked at me for a good few seconds, responded "we did" and walked back inside.
I still get filled with absolute shame whenever I think about it.
I was young and a bit of an idiot looking back, but I insulted him so badly, I still burn with the shame and feel absolutely awful every time I think of it.