For me it's a BASIC one-liner which generates mazes
10 PRINT CHR$ (205.5 + RND (1)); : GOTO 10
I found this specific one via slashdot[1], but something similar, which I've never managed to find/replicate, was used to generate mazes on the Atari 800 XL at my school when I was a kid.
[1] https://developers.slashdot.org/story/12/12/01/1847244/how-d...