It Chapter 2: 15 Insane Hidden Details You Definitely Missed
2. "Big Is What It Seems"
During the climax, Bev ends up trapped in a projection of the bathroom stall that she took refuge from bully Greta Keene in in the first movie. There, she finds herself tormented by It's projections of Greta, Mr Keene, her father and Henry Bowers and then almost drowns when the cubicle begins to fill with blood.
But before that point, we see her sitting on the toilet trying to block out Pennywise's influence on her and if you look at the wall of the cubicle, along with the note she wrote about Greta in the first movie and the Hate Clown tag (plus a hidden Pennywise cartoon), there's also a graffiti tag that gives the ending away.
It seems that It gave Bev the keys to its destruction - even subliminally - by writing "big is what it seems" on the wall. Given that It is destroyed when Bev suggests belittling it to shrink it, it's a clever little bit of foreshadowing.