Among other pieces of old hardware I acquired recently was an HP 700/96 terminal.
Works perfectly, but it's a bit dirty.
Started taking keycaps off to clean them.
The second one lost the little plastic nub that stops it from popping off.
That little dip in the rim is supposed to be flat, with a small protrusion that helps the keycap to click into place.
Really annoyed now - it goes on and works, but falls off at a moment's notice. Replacement keycaps for this aren't, and anyway they'd probably break if pulled off another keyboard anyway.
So I'm resigned to using a dirty keyboard with a loose keycap.