I have to admit, this is one of my dad's - and I'm a little rusty on details.
On a colleague's Unix machineof a colleagues, he injected some code into something related to the keyboard - a driver, or some handler for the input somehow. Basically, any time this person typed beyond about 45 words per minute, subtle typos kept being added; only keys added would be close to the key being hit. A would become an S or a Q or a Z, a Y ma may become a U.
The cruel part was that it would only happen if you typed quickly - if you backed up and typed it again slowly it would work perfectly. The faster one typed, the faster the errors came.
