Weirdest one I've heard of was one where the PC sort of ran backwards. I can't remember the exact details but it was something to do with an interrupt occurring

Weirdest one I've heard of was one where the PC sort of ran backwards. I can't remember the exact details but it was something to do with an interrupt occurring at the right (slightly-changing) frequency that it would get through an interrupt handler at a decreasing rate before being interrupted again, effectively executing less and less code each time. Something like that, it was some decades ago.