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 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: After the banker asks Toby if there is any inappropriate behavior, a few seconds later there is a flash back of Season 3 Ep. 22 when it is the beach games and Michael is hosting a hot-dog eating contest. Michael says to Phyllis to "Dip it in the water so it'll slide down - your gullet more easily." In the flashback version, everyone says "That's what she said." In the real version, no one says anything. Not to mention Toby wasn't actually at the beach day, Michael told Toby to stay because "Someone needs to stay at the office."
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.
Plot hole: Dwight is seen walking out of the bathroom with the diaper in the direction of the main part of the office in which Angela sits. If he was to try to stay hidden (which was obviously the whole point), he would have gone through the exit of the annex and Angela never would have seen. This was done because the plot required for Angela to notice but Dwight could have easily kept it hidden.
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: Nellie and Pam are driving along, discussing what to make up about people's ancestry. Here, we see the same background go past Pam, a driveway with a black SUV on the right, and a white SUV on the left. This shot is only repeated in the angle where we can only see Pam, and the view out of the passenger side window. The car is driving along as normal, we see the two cars on the driveway, and then the car turns a corner, and we see the same shot all over again.
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: 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.