“Why didn’t we just bop him on the head?” said the Dean. “Just a tap on the noggin? Then we could have dragged him back here, end of problem.”
“But it’s quantum,” said Rincewind. “We have to deal with what’s happened. IF we stop it happening before it happens, the other things we’ve…” he hesitated. “Look, it’s quantum. Believe me, I’d prefer it the other way.”

– look, it’s quantum |
Terry Pratchett, Ian Stewart, and Jack Cohen, The Science of Discworld III: Darwin’s Watch