The Orb at Glastonbury in 1993- I'd been drinking and smoking all day, and decided to do my first trip. Came up ridiculously strong during the Ozric Tentacles set, freaked out a bit as all these crusties were turning into vampires and had to go the side of the stage to try and calm down. I spent the entirety of the Orb set semi-comatose near some ditch, desperately trying to straighten out a bit. I also think I threw up on a tent at some point.
Not a single gig, but I lost an hour and a half during the Stag and Dagger festival in Glasgow last year. We started drinking early, and I stupidly couldn't be arsed to eat and ended up just staggering around various venues. I also took a header on the way back to the station when I tried to vault over a safety barrier- twatted my head and scratched the fuck out of my glasses. About the only vivid memory I have is someone asking me if I was OK, and me replying, 'no, I've got cancer'.