Baby, It's Cold Outside - S1-E18
Revealing mistake: When Wilson and Tim start to talk they sit down on some rocks. If you look at Wilson's foot you can see the sheet of fake snow caught under his foot.
Revealing mistake: Just before the dish explodes out of the dishwasher through the wall the cut out that was made to allow that to happen is visible.
Answer: No the guy sent in a picture of himself because he thought he looked like Al. So they brought him on the show as his brother.
Bowling255 ★