Mouse keys -- as an anti-feature. (press option 5 times)

My daughter enabled it while I wasn't at the computer and then thought my keyboard had died. Even took it in to get repaired, and got a new keyboard.

There is no permanent indicator that it's enabled, and it persists after reboots (once logged in)... when I figured out my brand new keyboard still had several broken keys I started looking for S/W level issues. Hard facepalm. It didn't help diagnosis that I did have some physically affected keys too.

At least I got a new keyboard out of it...