Back when I was young and started playing D&D it was not uncommon to bring your own D&D character to somebody's campaign. I took my character Lodor from campaign to campaign and it mostly worked out OK. He was my go-to character for traveling D&D starting when I was in 7th grade. I used him in my school's D&D club games, a home game, and a couple of other DM's games at school before he died in Tim McD's game and was subsequently revived in Darryl C's campaign. Most DMs back then, it seems, were OK with you bringing in a character from elsewhere. Most of the time they would just look over your character sheet and say OK. Sometimes they would put some conditions on your entry, usually just taking away some of your magic items if you seemed over-powered for their world.
Sometimes this led to problems. I related the sad tale of Lodor versus the DM's favorite NPC before. Not only was there a conflict between Lodor and Andemon, but Lodor really didn't fit in Tim's campaign world.
Oddly enough, Tim was DMing for the other worst case I can think of too. Tim was getting ready to leave for the army and wanted on last, great D&D game. He worked on the adventure for over a month and put together a pretty awesome adventure. It had a pretty standard "retrieve the lost relic" plot hook, but he came up with some cool and unique bit to go with it. The adventure had a cool sea travel portion, for which Tim had drawn deck plans on a one inch grid. There was an ancient red dragon. There was a pretty bad-ass NPC betrayal that caught us all by surprise. I was running my long-time character from Tim's campaign, Mandark the Wild; an 8th level Fighter. Scott W. brought his Halfling Thief character, Thorik, from my campaign; I believe he was also 8th level. Lance W. ran an Assassin he brought over from a campaign he played with Paul F. and Paul F. brought in his long time Halfling Thief, whose name I still don't recall. My dad played, in a rare appearance, his 7th level Cleric Kras.
The problem came from Darryl C. and his dad big Darryl bringing in their characters from their home campaign. Big Darryl brought his Fighter Royce Cea-Thor who was 6th or 7th level. Little Darryl brought his Fighter Borg's henchman Elisha, who was maybe an 8th or 9th level Magic-User, because he saw we were pretty good for Fighters and Thieves. Their home game saw significantly more magic treasure than any of the games the rest of us came from. Tim noted this and made some pretty drastic cuts to their items list. They actually negotiated over this for a while holding up the start of the game. They had pages of items. Tim should have cut their items worse than he did.
When the game got underway they managed to make pretty short work of the first few encounters, partly because Tim was unaccustomed to DMing with a Magic-User in the party, but mostly because they had a bunch of magic items still. They were used to burning through a lot of one use and charged items in their home game and were still a little resentful about having their stock cut so harshly. Tim was obviously starting to get annoyed at the power level of the two Darryl's characters. He started ramping up the encounters some to make them more challenging and that led to the rest of us getting spanked pretty hard. Still the game was a blast and we were mostly having a good time, although Royce lost an eye at one point while we were trying to figure out how a magic map gem worked.
Enter problem number two- Lance brought the Assassin, posing as a Thief, in specifically to take out Royce. He claimed that it was more or less an assassination on spec. See, I had had a character that Royce had cut the hand off of recently. He did it to rid me of a cursed sword, but the character was pretty much ruined. Lance had played a different character in that game and didn't much like the way Big Darryl took over the party and decided that Royce had to go. It kind of became a multi-planar grudge match. I wasn't completely innocent because when Lance and I talked I was still pretty pissed about my character's early retirement due to disability.
End game- We had a pretty bad fight against a Naga. We found the missing artifact. I set off a trap, hurting myself and the rest of the guys in the room with me. Then the NPC betrayed us, nailed us with a pretty high level fireball, then teleported to escape. This was bad enough. Sadly, this is when Lance decided to make his move. He figured Royce was low on hit points, maybe single digits. He took his shot and missed. Royce counter attacked and killed him. Then, a very pissed off Big Darryl had to be talked down from killing all of our characters. He accused us of inviting him and his son to the game just to kill their characters. This was maybe the last time any of us did any cross campaign traveling. The people from my two different friend groups never played together again, with a few exceptions, and most of those not terribly successful. I never really understood why I couldn't quite mix my friends together and have us all be friends.
In retrospect that fiasco was caused by the radically different play styles between the groups and it is a testament to Tim's skill as a DM that despite everything interpersonally that happened between the players and the characters that everyone who played remembers that adventure fondly. I feel that it is also noteworthy that this took place in the summer of 1985. I am not really sure why, I just have a feeling that there was a sea-change in D&D and how it was played that year, an age came to an end. After that, pretty much every DM I knew, myself included, insisted on players making new characters for their campaign worlds. One shots got pregens.
Why did we stop having world hopping open table games?