We just gamed tonight, in a campaign run by my son that takes place overlapping in time with the previous campaign he ran. Tonight the two campaigns merged, in that we met up with two of the PCs from the former campaign - only he's set things up such that we're allied with the major evil organization our last-campaign heroes were fighting, which put us at odds against the two prior-campaign PCs that showed up. He also figured out exactly when this adventure took place in the prior campaign: at a point in time where two of the players didn't show for the game session, so the other three players ran their PCs through a "steal a book from an arcane library" adventure. We now know why the other two prior-campaign PCs weren't there with the other three: they were busy fighting our "current campaign" PCs in tonight's adventure!
Johnathan