My first band I was in: we were a pop-metal band that played covers and originals. All the bandmembers except me were hardcore metal lovers, though we played soft-rock. Our original numbers were very poppy, but with long metal-style solo's

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Anyway, at a certain point the lead-singer/lead guitarist's wife got us a gig at the school where she was teaching. When we were there she expected us to dress up in big cartoon-character costumes and play like that for +- 10 children who were just running around and not really paying attention.
The whole band wasn't really to fond of this obviously, but I was the only one who spoke up. She didn't like me very much anymore after that. VERY Spinal Tap ...
In the cover band I am currently playing bass, we were setting up at a wedding, under open sky. Beautiful weather, a smooth jazz band with a great singer playing while we were setting up. Finally we are all set up and it's our turn.
Literally as we play our first note, out of nowhere heaven's flood drains open and from one second into the other it starts pouring. We all stop playing and together with the guest drag all our stuff inside in one big chaos. Meanwhile, apocalyptic downpour and thunderstorm.
That was one crappy gig ...
At another gig with a alternative jazz/blues band (originals) I played in, our guitarist kept getting electric shocks every time he touched his strings. We finally solved that by having the drummer and the guitarist switch shoes. Apparently the drummers shoes insulated better or something...
Also did a gig once with the same band where a local TV-crew was filming. At a certain point they pointed that camera with a big spot on it right in my face. I blanked from sheer nervousity and I think I just played random notes. Definitly the crappiest gig we ever played ...