The recent death of Bea Arthur reminded me of a topic on my blog from a few weeks ago, about a big yellow machine, and its place in my group's internal folklore. No, really, it did. The reason: somehow, Bea Arthur became a part of my group's folklore, meaning "a unit of length about six feet long". My memory of how this got started is hazy. I was GMing and describing a room, and something I said got misheard as Bea Arthur (I wish I could remember what it was), as a unit of length of the sides of the room. Given that Bea Arthur was unusually tall for a woman, the idea of a room being some number of Bea Arthurs in length is ludicrous but vaguely practical. And there's even a precedent.) From there it turned into a running joke, and then after sufficient repetitions of the joke, it became familiar enough that it almost lost its joke quality and just became part of the group folklore.
I can only think of a few more examples of this kind of local folklore unique to our group, before I start to get into borderline cases. The next most firm example is the twenty foot inflatable boulder. During an adventure on Hârn, our group was climbing high into the mountains, travelling through land occupied by barbarian savages on the one side, and garguns (like orcs) on the other. Unsurprisingly we had several combat encounters that took place on winding mountain trails in rugged terrain, and several times in a row, the combat map of a random spot on the mountainside happened to feature a very large rock at a curve in the trail, which either the attackers used to ambush from behind, or which we could use as cover. The vaguely peanut-like shape of the rock was even very similar in all three combats. We started to joke about how the ambushers lugged along an inflatable boulder to prepare for each combat; or that the propmaster for the movie of our adventure only had one prop boulder. Again, it turned into a running joke; every combat we asked where the twenty-foot inflatable boulder was, and the more inappropriate the better. And again, familiarity diluted the joke until it just became a catchphrase.
One that's quoted less often is the plan to "stab her in the eye and put the knockout patch on her", which alludes to one guest player who was in town for a week and for whom I wrote a one-shot adventure. Her character developed a stark animosity for one of the NPCs, so later when it was revealed that that NPC was one of the villains, the fight was on. Said villain had slap-on patches that applied a knockout drug transdermally, which everyone envied as a particularly effective weapon for time-travellers who didn't want to give themselves away. Our heroine was stuck with nothing but a knife and a seriously mean streak. On defeating, after a very cinematic hand-to-hand fight, the villain, and took her knockout patches, almost every other fight situation -- and quite a few non-fight situations -- led to the plan, "I stab him in the face and hit him with a knockout patch", often paradoxically in that order. Eventually it became a common first answer to the question "what do you do?" in almost any situation. And ultimately familiar enough that it needed no setup; just saying it was enough to bring up the joke.
I feel sure there's a bunch of other things like this but I can't bring them to mind. When I try, I mostly think of things that are more like inside jokes, but these feel like they've evolved past that to another stage... though maybe I'm just making that part up. I wonder how many a typical group has.