Sweet Johnny - S4-E5
Revealing mistake: The air mattress embedded in the ground that cushions their fall, and the curly hair and face of the stuntman for Earl are quite obvious when Earl and Sweet Johnny hit the ground.
Revealing mistake: Logan gets all the characters to spend spring break at his dad's house. When Zoey finds out Dustin drank all of Quinn's "fratz" she screams his name and runs to go and find him, but despite being meant to be angry you can tell she's laughing.
Saving Face - S3-E3
Revealing mistake: After they drop the casket and Provenza says 'guess you can take it with you!', the close up of the dead girl shows her close her mouth.