There is a soundman in Norwich who every musician knows as 'Deaf Geoff' if that's any indication of how awful he is! He has absolutely no idea what he is doing unless you're a standard guitar, bass, drums, vocals rock band. Acoustic instruments, you can forget it! He won't ever put any flute in the monitor so if you're playing with a flautist you can't hear them.
I was also invoved with a production of a musical where Geoff had to fill in for the other soundman who had set it up perfectly. Unfortunately Geoff decided to change it all which resulted in several very very loud booms from the speakers. Afterwards the Director went to have a go at him and Geoff blamed the booms on 'radio waves from taxi cabs'.
I also did a gig once with a 12 string guitarist who hit the strings so hard that they went more and more out of tune every second. It always took him hours to tune and the rest of the band had to sit round going 'no its still a little bit flat'. 5 years later after he'd played a few chords we'd be back to square one!