Homeless for the Holidays - S3-E10
Revealing mistake: When Mark goes to treat his cut, he leaves the lounge with a couple of tissues held to his head. He gets some supplies and puts a supposedly fresh piece of gauze to his forehead - however the gauze already has some "blood" on it - a feature the camera obviously wasn't supposed to catch. When Jeanie comes over to check it out, he pulls the gauze away from himself and - voila - there's the bloodstain.
Random Acts - S3-E20
Revealing mistake: When Dr. Greene is being attacked in the toilet, he is thrown into a supposedly brick wall. It bends and wobbles like a set wall would.
Answer: Take care of your dad.