Factual error: Twice in the film it is made clear that the Pym particle works by reducing the space between atoms in order to shrink an object, and by increasing it to enlarge them. This means that the object will weigh the same, whether shrunk or enlarged - it cannot be otherwise. A 90kg man the size of an ant would punch a hole through any surface upon which he stood (and couldn't ride ants), Doctor Pym has been walking about with a 60 tonne tank in his pocket, Darren Cross lifts a full grown sheep between finger and thumb, and the supersized Thomas The Tank Engine would be far too light to crush the police car (in fact it would float harmlessly away as it would probably weigh less than the air it displaced).
Factual error: In order to enter the Pym Tech building, they drop the pressure in the water main and Scott floats in on a raft of fire ants. Getting into it the supply line would mean they have to enter from the municipal main, which would be under full pressure. Even with reduced pressure, the interior pipe would still be full of water. Additionally, Scott would not be able to enter through a faucet unless someone had left it running (the valve would be closed).
Factual error: In the water pipe scene, water pipes don't have printing on the inside of them. Bolt flanges are not inside pipes, and welds are not that close together, nor are the welds inside the pipe. Also how did he get to the faucet without someone turning on the water to that particular sink?
Factual error: If the Pym Particle works by reducing the space between atoms to make an object smaller, the atoms themselves remain at normal size but are more closely packed together. It would, therefore, be impossible to "go sub-atomic" since the object obviously couldn't shrink and be smaller than the size of the atoms it was composed of.
Continuity mistake: When they are going over the plan for the last time at Pym's house a close up of Luis shows his top button undone, then the next close up it's done up with no time to do it.
Other mistake: Scott Lang is swept out of the tub after having shrunk for the first time. When he lands on the ceramic bathroom floor, falling roughly two feet, there is an audible glass breaking sound and it appears he has broken the flooring with his fall. He then falls through a plaster ceiling to free fall down to a turn table, landing on a plastic vinyl record played by a DJ. The album does not break despite him having fallen from three times the height and landing on a weaker material. Moments later, he is thrown through a second storey window and lands on a car in the street and his weight is now once again sufficient to form a small dent in the hood. (00:31:15)
Continuity mistake: When Cross is seen inside Hank's home, he is wearing black gloves, a few moments later, in the same scene you can see that he is not wearing them anymore.
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).
Factual error: When going sub-atomic, Scott passes into an atom. The image shown is of the Rutherford model of the atom with electrons orbiting the nucleus. This model is incorrect and has been replaced with the more-accurate Bohr model of the atom.
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.
Factual error: As they are heading back to the city, the scene switches back and forth from Luis to Scott. There is nothing but shrubbery visible through the window to Scott's right, but suddenly, there are two cuts in which they appear to be very close to an unidentified bridge. The closing shot on the scene shows the bridge to still be way off in the distance. The bridge would not have been off to their right if they were still approaching from the north.
Factual error: Near the end, Scott Lang inadvertently shrinks to subatomic size, drifting through a kaleidoscopic quantum universe, and we see him reacting to the visual effects. However, at subatomic size, Scott shouldn't be able to "see" anything, because the rods and cones in the human retina can only "see" in a narrow band of light frequencies, and Scott is far, far smaller than the frequency of visible light waves. Additionally, Scott couldn't possibly "hear" his daughter Cassie crying "Where are you, Daddy?" Human hearing is based completely on air vibrations at certain frequencies, and Scott is millions of times smaller than sonic frequencies or even the nearest air molecule.
Audio problem: Near the start of the movie, Darren Cross is showing the Yellowjacket armour to a group of some visitors. One person, named Frank begins talking with Cross about the armour. There is one shot shown during the talk, with the camera behind Frank's shoulder. In the shot, Frank is heard talking but his jaw is not shown moving along with the line.