I can do backing vocals to some songs but on others I can never find the right pitch. I just establish what I can sing to and shut up for the rest.
The club band that I played with for some years had a singer who was a right p!sshead. He would randomly swap verses and choruses, and start vocals seven bars into a 12 bar solo. The guitarist and I would just exchange glances and we (and the drummer) would follow. We kept getting gigs though. He used to ask if anyone in the audience would like to sing a song and then we'd be expected to play whatever the random drunk who wanted to sing fancied singing.
Mrs Zero had a problem with some of my songs, where the vocals start before the one (ie. at the end of the preceding bar), insisting that nobody else did that. Examples aren't hard to find though, and she eventually got it.
Tambourines - rule 1 is never leave a tambourine within reach of the audience. It's amazing how they can be heard over PA and backline, generally out of time. Just get rid of the sodding things.