Story I heard about mid-20th-century IBM mainframe. (I think it was the 360 series).
Core memory was tight and had cooling issues. The designers examined the instruction set and determined that, given cacheing and the like, no infinite loop could hammer a particular location more than one cycle in four (25% duty cycle), for which cooling was adequate. So they shipped.
Turns out, though, you could do a VERY LONG FINITE loop that hit a location every other cycle, for 50% duty cycle (not to mention the possibility of hitting a nearby location with some of the remaining cycles). Wasn't too long before a student managed to do this.
And set the core memory on fire.