Revealing mistake: Watch the line appear on the wall as Kelso cuts through the wall into the "Luxury Suite." The line is being cut with a round, drywall cutting bit such as would be used with a drill or Dremel tool. You can see it spinning as the cut is made. However, when Kelso has pushed down the wall and points the tool at Cox and the Janitor, he is now holding a standard reciprocating saw with a flat blade which would cut very differently than what was shown.
Revealing mistake: Right after Dr. Cox gives the results of the patients STD tests, the shot switches to show Turk, JD, the Todd and Dr. Kelso. Before the sprinkler activates you can tell that their hair is already wet from previous takes.
Answer: Dan was drinking beer and Cox feared that Dan might get so intoxicated he would fall asleep and drown in the tub.
Mortug