So, it was some 15 years ago and I was GM-ing this long-running, seldom-played campaign with a bunch of friends. We usually met up at the home of one of us, where we could spend a long weekend (or even a week) playing, during school breaks. As was, we usually spent the February "sports week" break, a two-three dayer during Easter break, a week during summer and a few days around Christmas. Needless to say, we were trying for Epic Things, including high-power NPCs to save the PCs' collective bacon, should things go too hay-wire.
Of course, all of us were involved with other gaming groups and such-like, so we had a tendency to have hangers-on during these longer gaming "sessions" (is it a single gaming session if you have sleep breaks?). One time, in particular, I can't recall where we were in the schedule, but I believe it was during summer, there were these two younger players (barely into their teens, IIRC) who were into RPGs who wanted to come along.
I was asked and said "Sure! No problems!" (after all, we'd had random players before, without too much of a hassle) "Make sure they have characters ready when we start." [ This was my FIRST error ].
So, they come along, happily dragging along their favourite PCs from whatever campaign they run normally. Fine by me, as far as I am concerned, anything their characters gain or lose (including magical items, life and similar) will reset at the end of our gaming meet. I glance over the character sheets and notice nothing too off-balancing. [ this was my second error ]
Now, we need to make a short diversion into Swedish gaming climate in the late 80s. The game at hand is a Swedish-language BRP derivation, it's a fantasy game (with magic and all that) and normally a magical item drains power points from its user. However, there is this one niggly little ritual that can be performed on the item when it's initially created (yes, even by a PC), but it involves permanent stats drain.
Suffice to say, I missed the fact that the witch's (one of the PCs) naginata had this no-drain feature, in addition to its "Does 6D6 damage" lightning bolt. "OOOOPS".
So, we set off for some skulking around ruins in the swamp. Every time combat happens, the witch's first instinct is to go "BZOT!" and one creature dies (with an average damage tolerance of about 12 and on average 18 damage, one bolt was usually enough). Now, I am a fairly patient fellow and I let this go on for a bit [ my third mistake ], until it's obvious that not only is my fun being ruined by this magical ack-ack, but so is the fun for most of the other players (the other young'un took a while to catch on to the fact that the rest of us weren't really enjoying having the tension of a carefully balanced fight being disturbed by a BZOT-BZOT (possibly a better term than ack-ack, actually)).
Eventually, it is time to infiltrate an underground temple, manned by smallish goblins (the sworn enemy of the 8 rat-men played by one of my regular players, think skavens, but with about 2 HP each, having 8 meant he didn't have to re-roll characters every fight and us finding an excuse for the rest of the party picking up more random strangers than usual, as it were). Tension is high, there are narrow tunnels and crossbow snipers hiding everywhere.
As the PCs are moving through the narrow tunnels, the bzot-bzot starts up at every hint of trouble, causing quite a death toll among the poor inno^Wrapidly being genocided goblins. All they ever did wrong was to defend the home of their goblin god's sacred temple from invading, obnoxious PCs.
So, the PCs manage to find whatever clue was hidden in the temple and start exploring for treasure and other Neat Stuff. Ms Bzot enters the holiest-of-holy, complete with a bronze statue of a demon, over an altar, obviously coated with dried blood (yep, this is the high altar).
At this point, I should probably mention that one of my players was an avid (and pretty darned skilled) miniature painter, so as they progressed through their environment, I drew a rough map of their surroundings and we used miniatures (25mm scale) to indicate who was where, up to and including all enemies.
So, Ms Bzot, obviously offended by the thought of blood sacrifices, decides to let loose at the statue. As it happens, I had a rough idea this would happen and had, on the off-chance, rolled up a suitably-large demon, to scare the shit out of any PC stupid enough to desecrate an altar in a game where almost-godlike beings were, shall we say, not unknown. SO, the statue gets blasted and I say "OK, there's this massive cloud of smoke that billows out from the statue and you can hear some rustling noises inside" and slap down an epic-scale Eldar Titan on the table (they're about 15 cm high, with 25mm being the rough equivalent of 2 metres, this means whatever's appearing is on the order of 12 metres high).
All PCs, bar Ms. Bzot, exit stage left as fast as their legs could carry them (it was mutually agreed that as long as I give sufficient warning, PC death could (and had) happen from combat or traps, though most of the time it didn't). Ms. Bzot, faced with a tableau of Ms. Bzot and this 12-metre MONSTER calmly says "I bzot it."
I look at the player and say "Pardon me? You are doing what?" (this being an obvious GM hint that, no, whatever you are doing is FUNDAMENTALLY stupid and I am giving you a chance to take back your stated action, because, no, it's so out there that had this been a film in a cinema, pop-corn and boos would start flying through the air). The other players go "What?" and look at me and Ms. Bzot's player. The poor young chap must've had a critical failure on some sort of inter-personal skill, because he looks at me and the figurines and goes "I bzot it!".
OK, house-rule time. If any PC or NPC lost half (or more) of their remaining HP in a fight, all their skill rolls were halved in the following round. This will soon make sense.
So, I look at him and say "OK, whatever. Well, you bzot it, the lightning bolt strikes it in the groin and it reaches forward and..." whereupon the player, by now familiar with the "half remaining HP" house rule goes "But? Don't I get to roll damage? What if it loses more than half?" and I say "Your maximum damage is 36, the point is rather moot." and continue with what I was saying. Roughly at this point, some wild backtracking happened. "Eh, no, actually, I ran out with he rest of them! In fact, I didn't even zap the statue on the altar! Nopes, not at all! But I was definitely legging it!"
At this point, the rest of the players point out that we, for two days, have used to-scale miniatures for everything. That all the players, except Ms. Bzot's player, realised danger was at hand and legged it and that Ms. Bzot was alone in a room with a BIG demon.
But, in the end, leniency won and the only thing the demon did was scare the living daylights out of Ms. Bzot (and ate her naginata, it's important having a snack, you know).
So, what did I do wrong? Firstly, I should have made it clear what rough power levels were expected (since most of our temp players didn't have characters prior to the game meets, we tended to roll them up and fudge them the night before).
Secondly, I should've paid more attention to the description of everything, especially magic items.
Thirdly, I should probably have rewound the game session and started from scratch immediately when I realised what extra features were on the offending magical weapon in question.
But, I must say, I think I did some things right. Firstly, I managed to remove the magic weapon WITHIN the game system, without killing any PCs.
Secondly, once the weapon was gone, I took the player aside and explained that, no, just because the item was gone "here", it wasn't gone once the character left my tender care and that the only reason it got removed was because it was ruining things for everyone else.
So, next time you invite a character from a different campaign, have a thorough read-through.