Other mistake: At the very end, Alec Baldwin drives away and other cars follow him. Problem is: two cars after Alec's car, the green traffic light turns orange, then red, and the cars keep coming. In all, 6 cars go through the red light.
Continuity mistake: During the scene when they are trying to bring Mr. Leniog into the rear of the door we hear them trying to open the door from the inside, shaking it rather violently. From the outside shots we hear the door rattling but it's not moving at all like it is from the inside shots.
Visible crew/equipment: Mini boom mike type thing appears in shot just as the real killer clown is walking onto the stage about to kill Kate. It is visible when he turns to face the 'audience' in the lower right hand corner.