Continuity mistake: The office in the book store changes from being quite large in the first season to very cramped and small from the second season onward.
Other mistake: The establishing shots of Buy The Book make it clear that there is an optometrist next to Buy The Book, but in the season three episode premiere Shake Rattle and Rubble, a major plot point is that there is a dry cleaners next to the store, which is not only wrong but impossible.
Continuity mistake: The bathroom in Ellen's apartment, depending on where the shot is taken, is either seen as incredibly small, or when shot within the bathroom, quite large and spacious.
Answer: This groundbreaking episode was in the fourth season, episodes 22 and 23.
Jason Feng