Two Flew Over the Cuckoo's Nest - S6-E7
Corrected entry: In the scene where Jack and Janet are sitting on the sofa with the doctor, Janet offers the doctor some nuts from a bowl. Then take a look at Janet's face. She begins to laugh so she lowers her head and the camera immediately shoots over to Jack who realizes the goof and adlibs something to give Janet time to get into character again.
Correction: On the DVD set of this episode (which I doubt is different than what aired, but I could be wrong), after Janet offers the "nuts", she hangs her head in shame. The camera does not shoot over to Jack, instead Janet goes into some dialogue about how she shouldn't have said said a thing and instead asks if he'd like cheese. During this time, Jack says nothing to either of them until he tells Janet "Good job!" for getting out of the situation, and tells Janet to not offer any more nuts and to not offer crackers.