The Puget Sound ferries often have a partially-done jigsaw puzzle on one of the tables. You can't finish it in 30m, so people come and do their part and move on. Eventually someone will put in the last piece, I guess, I've never seen it happen.

My last visit to Seattle was in 1998 so I can't confirm this firsthand, but I would bet that when someone finishes the jigsaw, ferry staff bring out a new one.