How about the one who waits until he gets on stage to start tuning?

That's not so bad, it's the one who waits until he gets on stage, gets his guitar out of the case, and then starts tuning, that p*sses me off :-)

Not long ago I saw a guy get his guitar out of the case, tune up (using an electronic tuner), which took ages, start playing, decide his fingernails needed filing, got the file out of a bag, filed his nails, then started from the beginning again. All in front of the waiting audience. I was sorely tempted to go to the bar during the rest of his act...