what do you do when the band your in starts playing different tunes?

This is one of those 'what-ifs' you talk about in advance... gotta have a plan what to do or whose lead to follow in case you start playing the wrong tune, or in the wrong key, or if the singer forgets the lyrics, or a million other things that are inevitable when you're onstage. Ideally, members should know each other well enough to instinctively act together to recover from an error and slide through it, and laugh about it later! Most important thing to remember is don't panic, it's not the end of the world because everyone makes mistakes now and then. Communicate!

That said, I went to a concert once and the band started asking the audience for requests. People started hollering their favorite songs, and when the band started playing, each member played and sang a different song, it was a hoot! Of course, it was on purpose.... but it was still hilarious.

:)

slap them and tell them grab their cajones turn up the distortion and let the curse words fly like mussolini from the balcony

Well, I would try to compramise as best as i could...

yeah, try to compromise, if that doesnt work, then leave