Many years ago I was at a session in O'Riordan's in Brentford when the guitarist who was there before me obviously, from the expression on his face, didn't realize that tunes could be in any key other than A till I strated playing.

One problem I've had is people who thing "Oh good, a blues in E", so nowadays I always announce that I only play thirteen and a half bar blues, though rewcently I had to ask people not to play chords when I was playing slide, as, although I was following the standard 12 bar progression, because of the slide I was playing lost of notes that didn't.