Picture Day - S1-E6
Continuity mistake: I'm not sure what type of mistake this counts as, but in here it is. In the episode "Speaking of Cancer" in season four, Jennifer reveals that she has been lying to Jack and Katie about them all having a shellfish allergy for years because it costs too much for them all to eat it, but in the season one episode "Picture Day" Dave puts down a plate and Jennifer says "Ooh are those crab-cakes?" and takes one to eat. Jack was sitting next to her, so he should have heard this and tried to stop her.
Bad Hair Day - S1-E1
Continuity mistake: The cookies that Jack is counting goes from a pyramid of 3, to 4 cookies in a single stack, and then there are 3 cookies are in a triangle formation. (00:13:00)
Continuity mistake: Nurse is handed a stack of cookies which mysteriously disappear when it pans back.
Continuity mistake: Ryan wheels Katie away on her chair, but when she walks back into the room later without the chair, the chair is back at the table. (00:14:55)
