Continuity mistake: When Norville grabs Amy at the juice bar, there is a shot from behind Norville showing his noise maker is perpendicular to the bar. In the next shot, the noise maker is parallel to the bar. Later in the scene, the noise maker gets crushed under Amy's purse, and then reappears uncrushed next to her purse.
Other mistake: When Norville takes the blue letter up to the top floor, there are several problems with Buzz and the three columns of elevator destination lights that he operates. When Norville enters, he is the only person on the elevator, but another floor is already lit up. When the next three employees enter the elevator, this other light has moved from 7th highest to 3rd highest in the left column. Two of the three employees are going to floors 11 and 37, yet Buzz only lights up floors in the right column (with three columns, each light in a column should differ by three, but 37-11=26, and 26 is not divisible by three). When Mr. Greer enters the elevator Buzz pushes at least two more buttons because when the elevator stops again, we can see at least seven floors lit up.
Continuity mistake: When Norville stands up and pays for a coffee after reading the job listings in the newspaper, you can see a tall glass container is an arms distance from the nearest two ashtrays. Yet a few seconds later, when we hear Tim leave the cafe, the glass container is next to both ashtrays.
Other mistake: When Norville first mentions his invention (you know, for kids!) to the elderly mail sorter, he shows him the paper with the circle drawn on it and he says "Yessiree this is my ticket upstairs" As Norville says this, the lower part of his face is visible at the top of the screen and you can see that his mouth does not move. (00:15:10)