Undo

You know you're spending too much time on computers when you are editing a document on paper and you remove a "do something here" flag, realise you shouldn't have, aren't sure exactly where it's meant to go ... and you reach for the CTRL and Z keys to UNDO.
I'm not sure where it was but best "undo" I remember is in an office missive encouraging people not to pass on chain emails especially bogus virus alert warnings because it put "undo pressure on the email system".