Spoilers for 13 Reasons Why Season 4, Episodes 1-8. In Episode 2, a new mystery was introduced to 13 Reasons Why fans. Someone spray-painted "Monty was framed" on the school doors, and Clay is suspicious of everyone — including his own friends. But in Episode 8, it's revealed that Clay is responsible for the 13 Reasons Why graffiti, a plot twist which was hinted at all season.
Clay is not the initial suspect. In Episode 2, the cops check Tony's bag for paint since his fingers have red marks on them. But Tony claims they are just from the auto body shop he works at. Clay suspects Zach (because Zach has become disinterested in school and somewhat of a trouble maker) and Winston, because Winston knows that Monty was framed. By extension, Diego could have done it, since he also thinks Monty was innocent. And Clay's girlfriend Ani thinks maybe Monty's sister Estela painted the words, especially since she had access to red paint in the room where the Hands Off Our Body club meets. But it wasn't any of them.
In Episode 8, Clay sets fire to the principal's car during a protest. But Clay has no idea that he committed arson — until his therapist shows him a video where he lights the car on fire. Realizing that he blocked that chunk of time out completely — in what seems to have been a dissociative episode, though it isn't formally diagnosed — his brain suddenly puts together pieces of the other recent unexplained events in his life. In a flash, he plays them out in his head and realizes he's also responsible for breaking the surveillance cameras at school, terrorizing his friends while camping, and, yes, spray painting the school doors.
Clay may not have realized it until eight episodes in, but the clues were always there. Shortly after the graffiti was found, Clay opened his locker and saw that a red smiley face was painted on the inside. He thought someone was trying to frame him for the graffiti — especially when he found an unexplained can of spray paint in his backpack — but it was just Clay all along. Liberty High's principal had also been suspicious that Clay could have been responsible, as he received an anonymous tip that Clay was involved. Clay responded that it didn't make sense for it to be him and the principal ultimately agreed.
The Clay we know has no memory of committing those deeds, but a version of himself did — seemingly the subconscious version that can't keep his secrets repressed anymore. So he spilled the secret that Monty was framed via that graffiti. And now that the principal has the car destruction on video, Clay can no longer hide from his actions.