In my roleplaying group, the phrase "big yellow machine" has come into common use, having a meaning somewhat like "red herring" but specifically referring to things the GM puts into an adventure just to screw with and mislead the players, as in, "Well, it might just be a big yellow machine."
A few years ago, while I was GMing, the player characters were crossing a huge, mostly empty, snowy field. (They were heading for Santa's Workshop so they could rescue Santa Claus from Santa Claws and his Men In Red, but that's another story.) In the middle of this trackless waste they encountered a big yellow machine. It was wheeled and about the size of a truck, though wider than long, and had lots of hydraulics, hinges, flanges, and fiddly bits which I described in elaborate detail. Though the paint was flaking in spots, it was painted in a sort of darkish yellow.
The players spent maybe an hour going over it, trying to figure it out. Who had made it, for what purpose, and why was it left here? What did it do, how did it work? Could they make some use of it? They tried everything and eventually decided they would have to come back to it later, so they continued on to the workshop, with a note to return when it became clear what significance it had.
Several play sessions later, the adventure ended, the characters dimension-hopped, and one of the players asked me what was with the big yellow machine. And I finally was able to break my straight face and tell them.
Jump back about three years earlier. On a spring or summer day, Siobhan and I were heading somewhere in the car. At an exit on Interstate 89, sitting in the middle of the wedge-shaped bit of land between the Interstate proper and the exit ramp, there was a big piece of construction equipment, of the kind they use to do road work. It wasn't anything familiar, though, like a steamroller, a mixer, a dump truck, etc. It didn't even quite look like the machines they use to score up the road when they're scraping, though it did have a bunch of downward-pointing curved flanges.
As is my wonted way, I wondered aloud what the machine was, and also, what it was doing there, in the verge, nowhere near any actual construction that was going on or even planned. Siobhan was amused at me wondering, and (her protests to my previous blog post notwithstanding) made fun of me for wondering things like that. By way of rejoinder, I commented, "If something like that was sitting there in a roleplaying game, the characters would be all over it trying to figure it out, even if they had something else to do or somewhere else to go." She seemed dubious.
So I memorized what the machine looked like, and it lay in wait in the musty corners of my brain for years until I was sure she'd forgotten all about it, and then I sprang it on them. Keeping a straight face through that entire scene was very very hard. A few times I had to excuse myself, pretend to go to the bathroom, and sneak in a few quiet chuckles.
(Note: the picture above is not the right machine. If I could find a picture of the right machine I could probably find out what it was!)