This has happened to me twice now. Playing in front of an audience we just finished playing one number and I go to kick the next song off. Were not making this up,we've practiced and the tune is in "G" ,at least that's the way we always played it. The previous song the guitar player(first time it was a banjo) used a capo and has forgotten to remove it. The resulting cacophony in front of an audience is really embarrassing to me. The first time after the shock and the moment or two that it took me to realize what was going on I changed up and played through in an unfamiliar key with the result of a completely awful performance. This more recent time I just stopped, asked the guitar player what key are we playing in and he sheepishly removed the capo. I tried to turn it into a comical moment ala "Smother's Bros." "Alright then lets try that again!"
Got some good embarrassing moments you would like to share?