Back in the day keyboard and mouse were connected to the back of the computer with identical (but color coded) PS-2 plugs.
Today one would expect that swapping two mechanically identical plugs would result in no major trouble - think USB - but back then COLOR CODES HAD TO BE OBEYED!
Or computer science teacher came to help when our PC won't boot up, tried reset, tried reset again, fiddled with the BIOS, gave up.
That wasted more than halve the lesson's time just for us to swap plugs back to working while he wasn't watching.
Sometimes things simply don't work when the teacher is around...
To be fair -- the error message right after the POST which was produced by switching PS-2 mouse and keyboard was more than cryptic and gave no indication what so ever on the real reason.