It's always a good idea to have an extra cable or two, just in case. If you're doing an open-mic / open jam kinda deal, you never know what kind of shape somebody else's gear is in. When jamming a common song with people I've never played with before, I'll also be sure to kind of chat with the other musicians beforehand to make sure we're all on the same page in terms of tempo, structure, and the version of the song (whether it's the commonly heard live version or the album version, ect.)
Also, for me, I try to just be sure I'm listening to what other people are doing to make sure I'm not over or under playing, which can be tricky (sometimes futile) in open-jam scenarios.
Another thing - and this one kind of sucks, but unfortunately it happens - is keep track of and protect your gear. A lot of times, these open-jam scenarios are golden opportunities for thieves to make their move when people aren't watching. And if it isn't a thief, it's some other inebriated jacka$$ who thinks they can just pick up any guitar that's sitting there because it's an "open jam". It sucks that it's that way, but I've heard of both cases happening to multiple people.
More than anything, though, HAVE FUN and learn as much as you can from the people you're playing with.