Visible crew/equipment: When Danni and JD fall out of the tree you can see the safety mat they land on.
Other mistake: The second '15 second countdown' with Jill Tracy, just after she's been to the toilet, actually lasts 24 seconds. They slowed it down to fit more dialogue in.
Continuity mistake: During the scene where Ted is explaining to Dr. Kelso about the stethoscopes being too tight, Ted is holding one in his hands. When seen from Dr. Kelso's point of view, the earpieces (white) are close together but when seen from the side they are crossed far past each other.
Audio problem: When Turk comes in to tell everyone that Dr. Kelso has ruptured his eardrums, Dr. Cox starts to say, "Bob you stupid mother..." from the back view. Then it cuts to a different camera angle, and he is mouthing the words, "stupid mother" again.
Answer: Dan was drinking beer and Cox feared that Dan might get so intoxicated he would fall asleep and drown in the tub.
Mortug