Continuity mistake: At the school, when Scott's suit is malfunctioning and he's big in the janitor's closet, his head is tilted towards his right shoulder. When Hope is working on the suit, we see Scott's head is now tilted towards his left shoulder, even though he's stuck and wouldn't have been able to move around like that. But in the next shot it's back to his right.
Other mistake: Throughout the movie, Hank's lab in its shrunken form is constantly jostled and tossed around, but when it's restored to full size in a new location, nothing is out of place and everything still works perfectly.
Continuity mistake: The outside of Hank's lab changes in different shots - sometimes there's a "no entry" sign inside a window on the right, other times it's missing. When Scott and Hope arrive there's a red logo on the door which is missing later. And when the ghost exits there's a "private property" notice beside the door which isn't there at other times.
Continuity mistake: When Susan goes to visit Burch and enters the building, she has the suitcase in her right hand but in the next shot she has it in her left hand. (00:21:45)