For me, it's the theory that in the original Spider-Man trilogy, Aunt May knows about Peter's secret identity.
I don't know whether the theory has been confirmed or dismissed, but there are quite a few rather obvious hints:
I like that it's not definitively mentioned in the movies, because it makes for a really interesting debate. I can totally see it being a complete coincidence and that she only cares about Peter and encourages him to be a good person – a hero, as she puts it –, which doesn't have anything to do with being a superhero. So in the end, whether Peter is Spider-Man or not doesn't matter to her. And that in effect means that whether or not she knows shouldn't matter to us.
That would actually be comic accurate, too (to some degree). At one point, Aunt May reveals on her deathbed that she secretly knew Peter was Spider-Man for a long time, and wanted him to know that she was proud of him before she died.
They retconned this, of course, bringing Aunt May back to life with no memory of Peter's identity. Then eventually did more stories about Aunt May learning Peter's identity, dying, then coming back to life... man, keeping up with comic book lore sucks.
Yeah I definitely agree with this one (but there’s definitely arguments both ways so nice for interpretation). I really enjoyed how May was basically Peter Parker’s (wo)man in the chair in ITSV.