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."