Continuity mistake: After getting fired from Baskin-Robbins, Scott goes "home" to the Milgrom Hotel. As he approaches, there is a bicycle chained to a parking meter in front of the hotel; it is also visible in the close shot through the gate from inside. In the long inside shot from down the hall as Scott actually enters, the bike is gone.
Continuity mistake: When Scott escapes from jail with the help of the ants, as he's flying through the trolley car he lands on a newspaper. From the position of the man's thumb, the picture is just a small part of the left side (his thumb lays over about half the width of the picture). When it cuts to the wide shot, the picture now covers more than half the page (width-wise).
Continuity mistake: Outside, when the tank hits the ground after bursting from the building, where did all the people who were being evacuated go? They were there in the previous shot, and since the tank landed on the path, Hank should have been in front, just behind or even on top of a few.
Continuity mistake: In the scene when Hank arrives at Pym Tech near the end of the movie, Paxton and his partner are sitting in their car. The left headlight is out. When the two exit the vehicle and go to talk with Hank, Gale steals the car with a working left headlight. Also when Paxton and his partner find their car crashed into a tree, the headlight is still on.