Revealing mistake: Everytime someone opens the front door to go outside there is just steps leading outside, but when they are on the steps outside and the doors to the inside are open there is a wall and a table with a lamp. Inside that is not there.
Revealing mistake: When Robert grabs Scott's shirt, Scott hits the wall and it moves backwards about an inch showing that it's fake.
Revealing mistake: Rudy snatches a hat off Kenny's head and she says the line: "Gimme back my hat Bud!" At same time, you can see the actor who plays Kenny silently mouthing the line with her. (00:12:50)
Denise Kendall: Babysitter - S6-E6
Revealing mistake: When the power came back on and everything started running at once, the food processor was still running after the lid blew off of it.
You Can't Stop the Music - S8-E22
Revealing mistake: Glue can be seen on Kenny's forehead after the carrot has been removed.
Some Gifts Aren't Deductible - S8-E23
Revealing mistake: When Stanley comes into the kitchen he has three demands, on paper from Deirdre, to which Kenny must agree. After Stanley reads the demands, Kenny signs it. You can plainly see that the paper is blank the moment after he signs it and gives it back to Stanley.
Chosen answer: That is the song "Open the Door Richard."