Fleetwood Mac (1972 line-up)

They tuned-up 15 minutes adjacent to (and ruined) a perfectly presentable band who had just played through a thunderstorm without benefit of the canopied stage allocated to Fleetwood. Then more tuning-up behind curtains, then more tuning-up without curtains. (30 minutes and counting - I was). Then they proceeded to chat in that arrogant knowing style with someone down at the front so no-one else really understood. They played one number of an indifferent nature and still strutted as if they were the bees knees. I never normally walk out if I have paid good money. This is the only time I can recall doing it.

AND don't even get me started on Steeleye Span.