(at around 55 mins) The piano girl is sucked into the bottom of the chimney feet first but when she is ejected at the top of the chimney, she emerges head first.
(at around 1h 25 mins) When Gary and Wyatt are going to say goodbye to Lisa, Gary's shoulder can be see reflected in the mirror Lisa is looking at, before Gary enters the room.
In Wyatt's bedroom, his Talking Heads poster is torn when Lisa is created. Later in the movie, the poster is intact, but then later it is torn again.
(at around 38 mins) 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.
(at around 8 mins) When Wyatt and Gary are inputting information on the computer to create Lisa, Wyatt's shirt goes from buttoned to unbuttoned (or open) in the following shots.
In the extended cut of the film, Frankenstein (1931) changes from its original black and white to its colorized version. Though, in the original theatrical cut it remains colorized.
(at around 17 mins) The fake driver's license Lisa gives Wyatt to get into the bar is cut crooked, the license number doesn't match the name, sex, and birth date information shown on the license. Driver's 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.
Gary says to the perfume salesgirl (Jill Whitlow) "one does not refer to a 23 year old woman (meaning Lisa) as a girlfriend)". Kelly LeBrock was 24 at the time of filming.
(at around 1h) A split second before the motorcycle crashes through the large window the pane shatters before the bike hits it.
(at around 1h 14 mins) 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.
(at around 1h 19 mins) 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.
Ilan Mitchell-Smith (Wyatt) clearly looks like he's about to start laughing almost every time Bill Paxton (Chet) looks directly at him.
(at around 5 mins) Gary and Wyatt are watching Frankenstein (1931) when they decide to create Lisa. However, scenes from the movie are jumbled around. Furthermore, the clip of Dr. Frankenstein (Colin Clive) saying, "She's alive!" comes from the sequel La Fiancée de Frankenstein (1935).
(at around 52 mins) When Ian and Max are talking to Gary and Wyatt in their bedroom, Max says "We'll let you have Deb and Hilly" and his voice clearly changes as if it has been dubbed from a previous take, plus it's also slightly out of sync with his mouth.
(at around 20 mins) When Gary was talking about getting kneed in his privates at the "Kandy Bar", Wyatt is supposed to say "worst pain there is" but his mouth stops moving after "worst pain".
(at around 21 mins) When Gary and Wyatt are leaving the "Kandy Bar" and he is giving money to patrons, pretty much nothing he is saying matches his mouth in the whole scene.
(at around 1h 17 mins) When Chet says to Lisa "I wanna get to the bottom of this, ASAFP", his lips clearly don't move when he says "ASAFP".
(at around 36 mins) 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.
(at around 54 mins) In the high resolution releases of the film, the equipment used when the furniture and the piano player get sucked out the chimney becomes visible. Casters are visible under the sofa, as are wires attached to tables, the piano bench, and the piano player's clothing. For when the actress is sliding across the floor, a harness is visible on her lower back, as is a cable leading from the harness to the chimney.
(at around 1h 18 mins) 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. However, the rear facing yellow lights should flash on a LAPD car. The ones shown burn solid.
(at around 21 mins) When Lisa, Gary and Wyatt are driving home from the Kandy Bar, they pass a sign for Lankershim Boulevard, which is a street in Los Angeles.