Revealing mistakes: When Chet finds Gary and Deb in Wyatt's room, he hits them in the head with the barrel of the shotgun, and as it's not actually made of metal it clearly bends.
Continuity: Gary's Cubs hat When they're using the computer to create Lisa.
Revealing mistakes: When everything is being "sucked" out of the house. The girl playing the piano is obviously being pulled by a cable.
Continuity: Gary's shoes at his headboard change positions between shots.
Audio/visual unsynchronized: The Porsche 928 sounds rather like the Ferrari when driven away from the mall - the engine of the 928 creates a much deeper sound, even under full acceleration
Continuity: Towards the end of the movie when Gary is driving the girl home, his sunglasses disappear and reappear between shots.
Continuity: When Chet is aiming his shotgun at Gary and Deb, his hands switch position between shots.
Factual errors: The fake drivers license Lisa gives Wyatt to get into the bar would be rejected by any bar bouncer in Illinois. Besides being cut crooked, the drivers license number doesn't match the name, sex, and birth date information shown on the license. Bouncers know that Illinois drivers license numbers can be decoded to yield that data. The number shown on the license should belong to a female born in 1951 whose last name begins with P.
Continuity: Right before Ian and Max drop the Icee on Wyatt and Gary, the cup is filled well over the top. Seconds later when they drop it, the level of the liquid in the cup has dropped considerably.
Continuity: When Gary and Deb are being chased by the cop, and cross the railroad tracks, the train changes direction between shots.
Continuity: Just before the "gun showdown" scene, Deb is wearing bright red lipstick. When the gunman lets Deb go, the lipstick is gone.
Continuity: When Gary and Wyatt are talking to Deb and Hilly in the bathroom, the red light changes to bright, dim and hardly seen at all.
Continuity: Lisa gets a beer for Gary and puts it down in front of him at the beginning of the scene when she meets his parents. A minute later, the glass of beer is gone.
Continuity: When Lisa first steps through the bedroom door after the boys create her, she's making a sultry pout. In the next shot she's smiling broadly.
Continuity: While Gary and Wyatt are watching "Frankenstein," the lines from the movie are changed around. And at the very end of the scene, "Bride of Frankenstein" is on the TV.
Continuity: The cigar that Chet is smoking is very short when he first blows the smoke in Gary's face, but is longer when he walks away and kicks his door open.
Revealing mistakes: During the police chase scene at the end, when the car turns one of the corners, it is very clear that the driver is not Anthony Michael Hall, but a stunt driver with a blond wig.
Continuity: Just before the Icee is dropped onto Gary and Wyatt, you see they only have a "large" cup. Yet when the Icee pours onto them, the amount is much more than the cup would have held.
Revealing mistakes: Ilan Mitchell-Smith (Wyatt) clearly looks like he's about to start laughing almost every time Bill Paxton (Chet) looks directly at him.
Errors in geography: Although the movie takes place in the fictional town of Shermer, Illinois, the police car that chases Gary and Deb is clearly an LAPD interceptor.