Agree with Steve, basically.

You just have to be able to adapt in these situations. No point in getting grumpy and complaining "they're doing it wrong" even although sometimes that's obviously the case...I can usually spot a different setting from obviously incorrect playing where it has just been picked up wrongly (We're all guilty of that, by the way).

Sometimes, it is a good idea to think of different settings as different tunes as Steve says. Often they may have different titles , in any case, even although the tune is bascially the same.

Different versions and settings can throw you, of course, as Jack says but if you play with certain people fairly regularly, you can usually anticipate what is going to happen. For instance, "The Barren Rocks of Aden" as I play it has four parts...I believe it was the Bobby Macleod setting.... although many people only play parts One and Two. However, in some sessions, they'll play Part one and then a hybrid of parts two and three... Very strange.