Has anyone ever crossed over two already running campaigns to make a super-campaign?
Our DM didn't turn up, for a very good reason that I can't divulge. Fortunately, we were playing a 14th century RPG, and our neighbouring game was Doctor Who, so wibbly wobbly timey-wimey ensued, character sheets frantically adapted, and our Knights (a handgun toting engineer, a lustful Knight Hospitaller, and myself as a "genuine" relic salesman and party leader) setting out from Jerusalem were teleported onto a massive Dalek battlecruiser in the middle of the Time War, with a group of badly endangered Time Lords standing around. One of them had just regenerated into an eight year old looking child, and was trapped under a dead Cyberman. Apparently, all these scientific geniuses hadn't the wit to use a lever to heave the cyberman off this hapless child. This done, the game began in earnest.
Several hours later, the Knights were transported via TARDIS back to Jerusalem. During this time, they had one-shotted a cyberman with their four barrelled handgun (markedly more effective than the Time Lords' lasers, cyber rifles, sonic screwdrivers and captured Dalek plunger), took an arrow to the knee (my relic salesman's aim was off), bartered a pig bone ("The Arm Bone of St Anselm") for the arm of a Lord of Time (the player had cut off his own arm during regeneration to get a second version of himself-like David Tennant in That Episode With Two of Him-unfortunately for him, he hadn't encountered Mr. Relic "I got the Knight Hospitaller to try and cure his venereal disease with a poultice of Siberian Wolf Dung" Salesman's capacity for spinning a tale), fried a Dalek's systems with holy water down the eye socket (which had been damaged, of course, by their handgun), inflicted repeated grievous bodily harm with the Hospitaller's double-bladed polearm, and (thanks to the Lord's Power via the relic salesman's crucifix) resurrected the Hospitaller... into a woman. His first action was to check himself out on a reflective surface. We all enjoyed ourselves immensely.