@kenshirriff at university I had an account on one of the first dual-processor Linux boxes. Of its CPUs, one had that FDIV bug, and one didn't.

That meant a userland process could detect when it was migrated between CPUs, by doing a hardware division operation that the two CPUs would answer differently.

The admin of the machine spent a lot of time in ytalks with Linus Torvalds, because Linux was just starting to develop its SMP support at the time, and he could provide useful statistical data!