Continuity mistake: Mary and Stan are lying on Joel's bed talking, with the plate of food between them. When the camera looks down on them, Joel's hand is on the plate. The shot changes at one point to a close up of Mary's face and Joel's hand is not there anymore, but reappears when the shot changes again.
Continuity mistake: When Mary and Stan are lying on Joel's bed and smoking pot, Stan is talking about The Clash and Mary is talking about how great their boss is. They are sharing a single joint. (The point is shown being passed back and forth.) However, Mary passes it to Stan. Then the camera focuses on Mary, who is now holding a joint.
Continuity mistake: Just before Joel walks in to see himself in the chair for the first time, he has a digital blood pressure monitor velcro'ed to his arm. Before he walks into the other room, the end of the strap is on the side of his arm facing away from his body. When the doctor takes it off, he has to reach around to the back of his arm to grab the strap.
Continuity mistake: When Clementine is getting ready to go out, Joel spreads ketchup across his neck and lies on the floor. Shots later, when Clementine has left, Joel sits on the couch and wipes it off, only there's no ketchup on his neck. Instead he wipes it off from his lips and cheek.
Continuity mistake: When Joel hides Clementine in a memory of him when he was four, he is hiding under the table crying and Clementine is trying to comfort him. The shot seen through Joel's eyes, while she bends over to comfort him, she tosses her cigarette away. The shot changes to behind Clementine, and you see her reach up to the table to put out the cigarette she already tossed away.