Re-tuning a guitar will do it almost every time, as will the band who swap instruments and need to check their tuning as well. I've seen bands who know they're only going to do a 20 minute spot at most still turning up with a zillion instruments which they insist on using, thus again cutting down time for other performers. I think a few masterclasses in "less is more" would probably be a good plan - it's rare to find established performers hogging the stage, in my experience.
And then the organiser/MC who burble on about forthcoming atractions and then remember the extended raffle which needs to be drawn...all of which is fine unless the pub has a definite closing time for the club room (or unless most of the audience need to rush off early).

Anne