My first ever gig with some mates from school, and being quite naïve about such things I decided that we were playing one song (I think Cake's version of I Will Survive) too fast, so decided during my solo introduction on the guitar to slow things down to the original tempo. I looked around before the main song kicked in to see the drummer absolutely livid with me mouthing "Speed up you wombler!". Not only that, but I'd been using a mate's Epiphone LP Special to practice (we already had a bass player so I was roped into learning the guitar), but during our soundcheck was told it sounded stinky poo by said mate's dad who was a pro studio engineer, so had to borrow the main band's guitarist's Gibson LP, which was my first experience with block markings on the fretboard and completely threw me so I messed up the solo at the end.
More recently I forgot that my current band does a cover of Let Me Entertain You in E (rather than the original F) and so was playing a semitone too high on that song for three gigs before I worked out why I sounded so terrible.