The fourth wall is an imaginary wall people break all the time because they didn't read the script of the episode they were in.
- Wait, what's my line?
- This show is a mess. I'm leaving. (walks out the screen)
- Sometimes, the fourth wall is shown on screen and shows cracking and/or breaking.
- And also, the wall will sometimes fall on the person/people who broke it.
- And rarely, the fourth wall appears again for revenge.