I was playing a 21st party gig and the band sounded awful. I suggested we all tune up before the next number...the two guitarists insisted they were in tune...it was obvious they weren't so insisted. Turns out the tuner the two were using was reading & displaying incorrectly due to a flat battery...this was mid 80s so tuners existed, were very expensive and prone to problems.
I managed to convince them that tuning to the same machine & testing the battery before doing so was a decent option.

It wasn't unusual that the lead singer/rhythm guitarist & the lead guitarist didn't know when they were out... for two weeks we rehearsed a song written by one of them and for two weeks it sounded awful. THEN they wanted me...ME to sing some backing part. They both insisted then both told me I sounded awful, was out of tune until I pointed out they were playing different, non harmonizing, chords...and that I was in tune with ONE of them, (NOT the singer by the way),then they started arguing between them selves about it. I didn't have to sing after that.

I have a bit of a cloth ear, I can't sing AND I'm a bassist but I could tell, almost every time, when these fellows were off.

Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe