The theory centers around Sean Astin’s Bob, aka the “Barb” of Season 2. In an early episode, good-natured Bob is trying to help Will deal with his nightmares, though at the time Bob has no idea of the truth of the Upside Down. In the car, Bob recalls his own reoccurring nightmare about a clown he’d met as a kid named Mr. Baldo. In his dreams, Mr. Baldo would offer him a balloon, and it wasn’t until Bob stood his ground in one dream and told the clown he wasn’t afraid that the nightmares stopped.
Sounds, uh, kinda familiar.
It’s not just that Bob’s dreams also involve a creepy clown. Bob is originally from Maine, where It is set. And he would have been a kid in the ‘50s, which would line up with Pennywise’s 27-year cycle of appearances. (The first part of the book actually takes place in the ‘50s, but the movie updated it to take place in the ‘80s.)
As to why Bob would be misremembering encounters with a demonic, shape-changing cannibal clown as mere nightmares? Well, in It, when people move away from the accursed town of Derry, Maine, their memories of the bad things that happen fade. They become almost dream-like. Perhaps this is what happened to Bob.