There are some very different ideas here..that's great...there are as many different types of musical event as there are musical events!

Etiquette is the right word I think. If you go to a session for the first time and want to sing or play then it is usually a good idea to do a song you are fairly sure is not a 'standard'. If there is another singer who seems to have a similar repertoire, then it would be a gracious act to say something like, "I hope this isn't one of yours!"

If you deliberately do other people's material, especially when they are in the room, then that says a great deal about you!!!!

Statute of limitations? As long as you credit the song to the singer you got it from you'll be fine. Just remember it will always be Ol' Fred's song....

Best wishes,

Peter