Continuity mistake: In the end of the movie when Farrah Fawcett is on trial, her fingernails are fairly short and not polished. Later when the verdict is read and she places her hands on her stomach, her nails are suddenly long and polished.
Continuity mistake: When Brian spends his first night in the underground monster world he goes in wearing a shirt and boxer shorts. But a later scene shows him wearing a shirt, pants, and shoes. He then goes back home and is back in his shirt and boxer shorts.
Suggested correction: It's entirely possible that he was able to borrow a pair of pants while down there.
I just watched the scene where Maurice and Brian pull the pranks to get the kids into trouble and he is in his pyjamas. Later in the monster world he is in a full outfit including a shirt, pants, and shoes. Then he goes back home and he is in his pyjamas again.
Continuity mistake: When Sam is swinging the shovel at Putnam's car he busts the driver's side headlight twice.
Continuity mistake: In the abandoned building in the jungle, a man is holding a chain in one hand and an iron ball in the other. The ball appears or disappears depending on the angle.
Continuity mistake: In the opening credits, when Smart gets inside the phone booth, the doors are not fully closed, but the immediate close-up angle shows them fully closed.
Continuity mistake: Ann visits her mother's grave at the cemetery with her father, who wipes off snow from the mother's photo on the marble. Couple of minutes later, a new shot shows the photo covered in snow again.
Continuity mistake: When Banner changes into Hulk in the subway station, his beard is gone. When Banner changes back after his "Hulk-out" his beard is back. (00:16:00 - 00:18:00)
Continuity mistake: The box of Marlboro cigarettes next to Buster's [Kiefer Sutherland] left hand did not have a lighter on top of it, but there was a lighter atop the pack after an angle shift. Buster was holding a newspaper with his left hand and had a cigarette in his right, so the lighter just suddenly appeared. Seconds later, the lighter was gone. (00:04:34 - 00:05:02)
Continuity mistake: In the beach scene the boys are eating ice cream cones. The ice cream keeps changing size.
Continuity mistake: John Ritter's beard goes from being real to fake throughout the movie.
Continuity mistake: Quick is wearing a grey overcoat and hat when he jumps from his car, but was wearing a white hat and overcoat when he left Calhoune's place and drove away.
Continuity mistake: When Harry throws back his arm to break the bodybuilder's front door with the crowbar, the front door and the object mounted on the wall are in the sun but as the camera changes angles where he actually finishes his swing and breaks the glass on the door, suddenly the door and object on wall are fully in the shade.
Continuity mistake: At the beginning of the movie (until Wasson's name appears in the credits), James Madison is writing on a paper copying from a large tome. Two people enter the room carrying a trunk. When he stands to go open the trunk and check its contents we can see that the book is gone from the desk. However, we see it again a minute later...when he picks it from those in the trunk and brings it to the desk. The first shot was a repurposed one. (00:00:50)
Continuity mistake: In the flashback scene when Eddie is at the beach it shows him talking to Sal. In the first movie he was talking to Frank.
Continuity mistake: When Lucas visits Meg at the juvenile home, he carries a stuffed toy monkey by the leg. When the camera angle changes, he is holding the monkey by the neck. (01:08:15)
Continuity mistake: When Angela, Snowboy and Peter are fishing, Peter puts a firecracker into the fish's mouth twice. (00:22:25)
Continuity mistake: When Tim gets stabbed, the killer's silhouette shows that he's holding his knife overhand. When he stabs Tim, he's holding the knife underhand.
Continuity mistake: During the graveyard battle, The Warlock throws Kassandra head first into a pond. In the next shot, she falls feet first. (01:31:45)
Continuity mistake: Barbara Carrera's Priscilla has shoulder long hair most of the time yet at one point she's sporting a ponytail that reaches her waist.
Continuity mistake: When Alex is speaking with Christopher in their dorm room she's wearing a pink top with the right shoulder down. Then it's pulled up in one shot before being back off the shoulder again.