GTA Online yesterday: I'm cruising around the desert near the Redwood Lights dirt bike track at night when suddenly out of my peripherals I see a minivan barrelling down one of the straights.
It wasn't a player, because there was no one on the map and I couldn't auto-target him when I hopped out of my own car, so it had to be a random NPC driver. It was, and it looked like he was in the midst of completing a lap, so I followed him - and sure enough, he did three laps around a freaking dirt bike course in a minivan, speeding and bouncing and drifting all the way. I could have tried to race him in my rally-prepped AE86/Karin Futo, but I wasn't sure what would happen if I accidentally made contact, so I hung back and enjoyed the show.
After the third lap, he simply turned off the course and trundled away at a normal NPC speed. Wish I had a capture card for proof. Guess they gotta have fun sometimes!