One of the locals does a jam night. Often, one particular band turn up, set up their PA and play loads of songs? Its a gig really. If anybody else wants a jam they get a bit miffed and dont like others using any of their gear (bear in mind they have taken all the available space for any more equipment up with theirs!).
If by chance somebody else gets the chance to play something, their guitarist who knows every song ever written quickly joins in (closely followed by his bass player and singer) and take over. Its quite ridiculous really. Whatever happened to 'lets play something in A'!