The text plays on the ironic twist of the phrase 'if all goes well,' which typically implies that everything is proceeding without problems. However, the addition of 'you've overlooked something' suggests that if it seems like everything is perfect, then a mistake has been missed. This is not a play on different meanings of a word or similar-sounding words, but rather an unexpected reversal of the expected outcome.
If all goes well , you ' ve overlooked something !