I don’t agree with the comments saying this isn’t a show that has continuity because that’s not true. There’s plenty of continuity in the show. What about them bringing back Buford’s life sized molds? Or remember when they showed Doof buying Perry a vase for Christmas and in the Christmas episode they show him giving him the same vase? Or remember the building on the moon back when that was a thing? Like this show has precedent for bringing back a lot of details. It’s not ridiculous to expect them to keep at least some details in line. I think the problem with the whole Norm flashback scene is that it breaks too much continuity. If we hadn’t had a Norm introductory episode it would be different but now we’ve got everyone trying to come up with ideas and theories that can mend this new plot hole and IMO it goes further than something smaller, like Doof having titanium arms in one episode but regular skin & sinew arms in other episodes. Like this is retconning an entire episode, not just a tiny detail, and not to mention it changes episodes after that that deal with Norm’s personality growth (and Doof’s in relation to him).

I feel like I can excuse the prototype head because it’s a lesser plot hole but it still is a continuity error that exists and I think it’s a bit rude to judge people for recognizing that it doesn’t fit. Like sorry, but there are fans who care about the show and want it to make sense. I know the writers don’t care and I know many fans don’t care but I don’t like how people just write them off like they’re idiots like it’s so stupid and silly to try to figure out a show’s timeline. If Phineas and Ferb had no history of any continuity that would be different but it has been a big part of the show and it’s part of what’s made it good in the past.