Continuity mistake: In "The Alliance" (1-4), Pam says that the next office birthday is Meredith's, but it isn't until the next month. Later, in "Survivor Man", it is said that Meredith's birthday is at the end of 'Birthday Month', when many employees have birthdays close together. It wouldn't be possible for Meredith's birthday to both be both the next office birthday but also at the end of the following month, which is stated to have many other birthdays in it.
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 this episode 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.
Continuity mistake: At the end of the episode, we are shown a video of Dwight getting splashed with the milkshake from the point of view of the guys in the car. But it splashes differently the two times we see it. The first time it hits his lower body, and on the video, it hits him in his upper body.
Continuity mistake: After everyone but Dwight and Kelly exit the building (because of the fire), Dwight picks up the water jug and accidentally knocks over the blue cups next to it. A few cups fall to the floor but one remains still on the printer (next to where the jug was). After Dwight gets Kelly out of the kitchen, the blue cup has disappeared from the printer.
Continuity mistake: When they're having the Diversity Day meeting Jim's chair can be seen over Michael's shoulder several times. In the beginning of the meeting the chair is pointed towards his desk, but when Kevin says that he remembers the Chris Rock routine and begins to recite it, you can now see that the chair is pointed towards Pam's desk; however no one has left the meeting in that time. (The only person who's not in the meeting is Devon. He was at his desk and walking around a few times but it makes no sense that he would go and move Jim's chair.) It's obvious that the actor left the chair in different positions during different takes.
Continuity mistake: While Michael is introducing Ryan to Dwight and the stapler-in-jello is found, we see that Phyllis' chair is empty. Then a moment later there's a shot of her looking at Jim while sitting in her chair with no time to get there. And then the chair's empty when shown again.
Continuity mistake: When Michael is having 'Harvey the talking computer' talk to Pam and Jim, the screen of Michael's computer is shown several times. At a few points you can see that Harvey's comments have already been typed out in full long before he said them to Pam/Michael typed them up.
Continuity mistake: When Jim and Dwight are talking about the activities for Kelly's party, Dwight is attempting (horribly) to blow up a silver balloon. Just after Jim finishes reading the list of proposed activities, Dwight says, "You're right, forget horse hunting, it's stupid." Immediately after he says this, the shot changes and he is now holding a brown balloon.
Continuity mistake: Episode 4 Season 1 "The Alliance" Dwight is hiding in a box in the basement when Pam comes down to the basement and pretends to make a secretive phone call. As she does so the box that Dwight is hiding in changes position on the floor, presumably so that the camera can catch the holes that Dwight has made in the box in order to breathe.
Continuity mistake: When Michael takes Katy (the purse lady) around the office, she worries about leaving her purses unguarded so while walking by Phyllis and Stanley's desk clump he asks Ryan to look after the purses, but Ryan says he can't because he's "installing File-Share on all the computers." Both Stanley and Phyllis are noticeably absent from the shot. Stanley's desk is empty and Ryan is sitting at Phyllis' desk. Michael and Katy take a few steps and walk into the kitchen area. In the background we can now see that both Stanley and Phyllis are now sitting at their desks and Ryan is nowhere in sight. There wasn't enough time for everyone to switch locations like that as only a moment had passed between the shots.
Continuity mistake: During the basketball game just before Pam throws the jump ball, Stanley is seen standing over Pam's left shoulder. After she tosses the ball up Michael hits the ball mid-air over to Stanley who is now about 10-15 feet in front of Pam, with no time to logically get there between shots.