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: When Meredith's hair catches fire, we never see her face (as it's a stunt double). After Dwight uses the fire extinguisher on her, the shot cuts (although she is still in the frame) and her hair is different, as it is now the original actor that plays Meredith.