Revealing mistake: When Kelso hits Dr Hendrick's bike with his car at the end, you can tell it is not Kelso. While you can't make out his face, you can see that the man driving has dark black hair, while Kelso has light brown/grey hair.
Continuity mistake: When JD opens the beer can with the "John Dorian three taps method" inside Dr. Cox's car, he holds it in the hand in different ways.
Answer: Dan was drinking beer and Cox feared that Dan might get so intoxicated he would fall asleep and drown in the tub.
Mortug