My favorite assembly foot gun was a guy I worked with had a cute routine. You had a call to the routine, followed by a null terminated string after that. The routine would spit the string to the terminal. And then return to the location after the string.

He had some bug where in one place it returned to the start of the string, executed it, and kept going. The end result just happened to be a nop. Had been like that in production for a couple of years.