When I was about 17/18 I was in the pit orchestra for ‘Cats’. It was my first time doing a paid musical and I really wanted to be professional. In one of the songs there was a repeated section of really quiet ambient music which then segued immediately into a loud orchestral hit.
Long story short, I forgot to play the repeat and confidently came in by myself with a raucous french horn triple forte stab note and everyone in the orchestra laughed at me and I could hear the audience laughing. I somehow managed to not get in trouble for it though, which was incredible!
Absolutely mortifying. Lesson learned: ALWAYS check for repeats!!