Continuity mistake: In one episode Angela says she hasn't talked to her sister in years over an argument that she doesn't remember. When she's engaged to Andy and they're touring Dwight's farm he mentions she has multiple sisters. In the finale she only has one sister and apparently they are incredibly close.
Continuity mistake: In 4-14 "Goodbye, Toby", Andy Bernard invites his parents to the going-away party for Toby Flenderson and introduces them as "Andrew and Ellen Bernard." But in 6-16 Andy tells Jim that he was originally named "Walter Jr." after his father and that it was changed when his little brother was born and the name Andrew was one that they got out of a baby-naming book. Additionally, in 8-4 "Garden Party", Dwight is introducing the entering guests. When Andy's parents arrive, Dwight says, "Mr. And Mrs. Walter and Ellen Bernard." (00:36:20)
Continuity mistake: During season 1, specifically episode six, it is shown that the show is filmed in a real office building. The windows we see in Michael's office and the conference room match the exterior windows as they all walk to their cars at the end of the episode. However from season 2 onwards the office is at ‘1725 Slough Avenue', featuring the building everyone's familiar with. However if you look at the windows on this building they do not at all match the set interior, they even have vertical blinds that are visible. A large set error they ignored when moving to the replica set.