We’ll start with Meredith’s birthday. One savvy IMDB user noticed that during season one’s episode, “The Alliance,” Michael is looking for the next upcoming birthday. As Pam points out, the next birthday is Meredith’s and it isn’t until next month. However, this contradicts with the information provided in season four, in which Angela reveals that “birthday month” consists first of Creed’s birthday, then Oscar’s, then Meredith's.
There also seemed to be some confusion regarding Michael’s siblings. In the series pilot, Michael references his brother (who apparently taught him the joke about a special filing cabinet “the trash” in which to file the memos from corporate). However, later in the series, it is revealed that he only has a half-sister named Marnie Cooper. So, NBC, which one is it?
And then there is the issue with Michael’s bike riding skills, per Screen Rant. During the cold open of season seven’s episode “The Sting,” viewers learn that Michael can’t ride a bike when he hops on Oscar’s and crashes into a car. However, in season three, the Dunder Mifflin manager rides into the office on a bike to donate to charity for the Christmas party.
Perhaps one of the most important errors we picked up on while watching the series is that the same actor was cast in two different roles for the show. That’s right—actor Nelson Franklin plays two minor parts The Office. He first appears in “Job Fair” as the guy who tells Pam that if she wants to get into graphic design she should look at programs in “New York or Philadelphia.” However, Franklin appeared again in later seasons as Nick who works for IT. This also happened with child actor Blake Garrett Rosenthal who was cast as Dwight’s nephew and as a kid participating in Dwight’s Hay Place fair. Maybe NBC thought we wouldn’t notice, but we did.