But almost no one really follows this advice to its core. Imagine an operating system designed this way: you could quit Photoshop, then "undo" the quit, and all your windows would reopen without anything have to re-load from disk (because it wasn't really unloaded yet to begin with.) You could "rm -rf /" and then press Ctrl-Z in your shell. You could send an email, and then pull it back from the server (or all the way from the other person's inbox), as long as they both had the same system installed.

If the entire world worked this way, you could even wire $5000 to a Nigerian scammer, then pull it back out, undoing the purchase of anything they spent it on in the process. Of course, cash would have to be replaced with a reversible commodity (credit chips that really just referenced a row in a central, though distributed, table) so money could flow back into your pocket without anyone having to hand you anything.

Taken even further, to the point where you can take out a hit on someone and then press undo after the person is dead, this begins to sound like the start of a good Science Fiction. "The Reversible Society", perhaps.