What are some documentaries where the filmmakers set out to document one thing but another thing happened during filming that changed the entire narrative?

I was telling my daughter that I love when documentaries stumble into something that they were totally not suspecting and the film takes a complete turn to covering that thing. But I couldn’t think of any examples where it did.

Pretty sure there’s a bunch that covered the 2020 election that stumbled into covering the January 6th insurrection. So something like that.

EDIT: Wow I forgot I posted this! I went and saw Avatar and came back to 1100 comments! I can’t wait to watch all of these!