Other mistake: Everything goes backwards in time and goes that far back that Meg (who is voiced by Mila Kunis) goes back to being voiced by Lacey Chabert (who voiced Meg in season 1). However, when Lacey Chabert does her line to Stewie being in the Griffin house for the first time, it goes back to Mila Kunis when Stewie is "reborn". However, if everything was going backwards and Brian fixed everything to make it goes forwards again, then it still should have been Lacey Chabert voicing Meg, not Mila Kunis. (00:16:30 - 00:19:35)
Continuity mistake: When Stewie is born whilst time is reverting, Doctor Hartman says "It's a girl... with a penis and no vagina". Then after Brian restored the flow of time and Stewie is actually born, Doctor Hartman instead says "It's a boy". (00:18:25 - 00:19:55)
Continuity mistake: In reversed time, we see Cleveland falling back to the inside of his house that had just been wrecked by Peter who launched himself with a cannon into it. The camera zooms in on Cleveland as he falls back to the inside of his house, only for Peter to suddenly be visible along with an additional piece of the roof taken away, after the area in which Peter would appear on and the roof-piece disappear at was off-camera for a few seconds.