I may have come across it, but it would have been resolved in episode 1 and then I'd forget the irrelevant character who I believed would be the main. Like if you thought Koby was the main character of One Piece because he's the first one shown and has dialogue and has an established plot (mistreated by Alvida), but then you find out it's Luffy and then imagine Koby never appears again. You might just forget about him (sorry Koby. You never knew what you could become), but he was the main character for 30 seconds. I could imagine a shows like Baccano doing something like this. Also Daily Lives of HS probably showed a few HS boys and you don't know who the mains are until later (if there are mains. MAL lists some as mains, but I feel like some of the non-mains ought to be "mains" on there too), though there I guess I didn't think of anyone as the main since nobody stands out - they were just potential mains. Anyway, I guess you're asking about times I thought someone's the main beyond the first episode or chapter.
I'm not gonna spoil it, but Danganronpa (video game that has anime adaptations. Games are way better though) pulls what you've described, in no uncertain terms. It was a big talking point, with people thinking it was a great writing but also some people being disappointed because they liked the faux-MC.
I actually sometime avoid watching the OP until I'm far into the series nowadays, because of this - it's nice to not know who the mains will be or what powers or outfits the characters may end up with. Eg two characters dislike each other, but from the OP you already know they'll team up, so it's not the same journey of wondering how the relationship will turn out (still, at least you still won't know HOW they become teammates. You only know that they will). Maybe you like character A and don't really like character B, but the OP tells you character A will be major and there's no showing of character B - it could make you less motivated to continue the show, before you get a chance to find other motivations to continue it (such as character B becoming more interesting to you, interesting character C joining, or an interesting plotline or visual style).
Bulma could've been one back in the day. For us who knew about Dragom Ball Z and Goku being the main, we're less likely to think Bulma is the main, but in the '80s someone could've thought she was, when she's just a very major character/secondary protagonist - which doesn't become clear until at least after the Pilaf saga (which is ofc when Toriyama originally thought he might end Dragon Ball). Wonder if anyone here will live long enough that they have to specify 1980s and not 2080s. Unlikely friends, unlikely....
Leorio, Kurapika and Killua I fully thought would be mains after the Hunter Exams of HxH. Even a few arcs later I hadn't let go of that idea. Gon ended up clearly being the focus, maybe with Killua too.
I could imagine romance anime/manga doing it. If a second lead ends up with a lead, is he/she now the "first lead"?
Moving is based on a manhwa and I thought the kid who gets beat up was gonna be a main (cuz he's the classmate of some mains and was good-looking), but he's just a side character.
Jojo would be one. It seems like the English upper class one will be the main, but then it's his descendents instead.
