[foxglacier]

Since this is an age competition, I can remember the annoyance of XT and AT keyboards and motherboards having the same connector but being incompatible. Some keyboards had a switch to change modes. I don't know what advantage the AT keyboard provided but it must have been huge to be worth breaking compatibility in both directions.

[eashman]

How about some of the PC “clones” that used older dumb terminal components and ports, probably to ease upgrades on accessories for some corporations.

Our computer lab in college had these NEC/AT&T 386 desktops that had RJ-11 keyboard ports that didn’t work anywhere else.