"Fly on the walldocumentary" meaning a documentary where the cameras go unnoticed by the subject of the film.
"I wish I had been a fly on the wall at that meeting" meaning I wish I could have sat in unseen on that meeting.
The fly that landed on Mike Pence's head during the Vice Presidential debate of 2020. It sat on his head for approximately 2 minutes in which everyone thought it died, but it flew off. This caught a ton of media attention as Mike Pence was seen as a piece of shit, due to him being affiliated to Donald Trump, the 45th President of the United States.
Person 1: Bro! Did you see the fly on Pence's head during the VP debate? It sat there for like 2 minutes.
Person 2: Lol yeah! I mean it makes sense, because Pence is quite the scumbag, like his partner, Donald Trump.