In this clip the nurse gives the old lady something to drink before the X-ray and asks her to drink it all. "What's it for?" the old lady wonders "Well, it's so that we can see your insides in the X-ray" the nurse responds. The old lady takes a sip. "You should swallow" the nurse says "What?" "You need to swallow" -"That's what he said last night!"
.... the crew can not contain their laughter. In fact, only the two actors knew that they were going to do this joke so it really did take the crew by surprise.