Continuity mistake: In 2-13 "The Secret" Pam and Jim are talking about how Jim used to have a crush on Pam when she first started here. This implies that Jim was working here when Pam was hired. However, in this episode Pam says that when Jim first started working there she said, "Enjoy this moment, because you're never going to go back to this time before you met your desk-mate, Dwight." These two scenes contradict each other.
Revealing mistake: In the first scene, everyone is in the conference room, just waiting for the block on the TV to hit a corner while Michael is blabbing away. The screen has no reflections, but later when Michael talks to the camera there is a reflection. In the scene you can also see the off shading on the characters in it, making it very apparent that the scene was filmed using a green screen.
Factual error: Michael and Jim are driving from their Scranton, Pennsylvania office to the company's headquarters in New York City for a website Launch Party. However, during the drive, it is clear by looking out the car windows at the mountainous and desert landscape that they are in California, where the show was largely filmed, and not in Pennsylvania, which does not remotely resemble that type of geography.