[gattilorenz]

Big parallel ports? There was some proprietary DB-something connectors (for example, the Olivetti M24/AT&T 6300 had one), but if you're younger than 40 you likely have been using an AT connector (basically PS/2, but physically larger) for the keyboard, and a DB-9 or DB-25 for the serial mouse - and that is easy to mistake for a parallel port.

PS/2 and AT refer to the IBM computer models where the connectors were introduced, of course.

[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.