Other mistake: When Arnold and Gerald first bump into Helga at the bus stop, they ask her for bus money, but throughout the rest of the movie they never need bus money.
Continuity mistake: When Big Bob throws a model of Arnold's house in the trash it lying on its back but when it shows it again it is standing up straight.
Continuity mistake: Inside the FTI records room, Scheck tells Arnold and Gerald, "My guards will now lock you away where you won't cause any more trouble." As the FTI guards approach the boys, Gerald notices the two crisscrossed cutlasses hanging on the wall, with scabbards of different colors, but when Gerald reaches for those two weapons, the two cutlasses are facing the opposite way. Additionally, the lighter scabbard tip changes color, and the hooks they hang on also change, from transparent to silver. (00:51:50)
Continuity mistake: Scheck calls Nick and informs him the kids have a copy of the document on videotape, and they're headed uptown. During the following scenes, both Nick and Big Bob have medical adhesive tape on their cheeks (due to their earlier fight). In some shots Nick's cheek has two crisscrossed tape strips, but in other shots only a single strip of tape; the changes go back and forth. (01:03:20)
Continuity mistake: When Murray is told it's Mona's neighborhood that's to be demolished, he begins to speed. Murray then tells the kids to pray for the hunk of junk bus to hold together long enough to get them uptown in one piece. In the next shot, the rear right tire's hubcap flies off, but in the following shots, that hubcap is back on the rear right tire. (01:00:35)
Continuity mistake: After Scheck catches the duo in the FTI records room with safe deposit boxes, Scheck removes a painting from the wall while he rages on about the tomato incident; then he destroys the painting. However, when Arnold fights off the guard with the mustache, that painting is intact and back up on the wall in this shot. (00:51:30 - 00:51:55)
Other mistake: While Arnold and Gerald are headed to FTI on the city bus, Murray tells them about Mona. When Murray says, "You meet a girl, you think it's forever," the animation is a bit skewed, as it shows the steering column going through Murray's right thigh. (00:46:25)
Continuity mistake: When Arnold and Gerald go to see Mr. Bailey at the Office of Information, Bailey's tie has diagonal stripes that change direction to the opposite way in some shots. (00:26:15)
Continuity mistake: In one scene, Grandpa packs up all of Arnold's stuff, but in the scene when Arnold and Gerald have a sleepover, all of Arnold's stuff is back in his room.
Other mistake: In the FTI records room, when Arnold and Gerald open safe deposit box 999, the box is quite empty with a mirrored surface, and we can see the reflection of Scheck appearing behind them. The problem is Scheck's reflection is not backwards. (00:50:50)
Continuity mistake: At the start, while the neighborhood crowd watches Scheck make his statement at the televised press conference, when Eugene begins to sing, mailman Harvey's buttons have switched from his right side to left side on his uniform shirt. Harvey's moustache has vanished in the interim as well (his white undershirt also disappears and reappears). (00:03:25 - 00:04:40)
Continuity mistake: Bridget gives the "synchronized, programmable, glow-in-the-dark watches" to Arnold and Gerald. When the two boys steal the safe's key from Nick, they wear their watches on their left wrists. Then, Arnold and Gerald get on the city bus, and thereafter for the rest of the movie, both of their watches randomly vanish and reappear during numerous successive shots, as well as mid-shot. (00:45:40)
Continuity mistake: When the police come and suppress Blockapalooza, the letters on the police van's license plate keep disappearing and reappearing. (00:13:40)
Continuity mistake: After Mayor Dixie reclaims the neighborhood as a national historic landmark never to be torn down, Phil and Bob go to pick up Arnold and Helga. In the next shot, the people standing around them change position. (01:07:42)
Audio problem: When the bus is heading towards the bulldozer, Arnold and Helga start to scream. They then close their mouths, but their screams can still be heard. (01:06:05)
Other mistake: When Bob and Nick start fighting, Nick runs into the kitchen and grabs an egg beater. Right before he tries to attack Bob with it, his right shoulder is drawn incorrectly. (00:49:32)
Continuity mistake: When the bus is about to jump the ramp, Arnold is on Gerald's left, and Helga is on his right. As the bus runs over the ramp, Helga is missing from the bus, and Arnold is on Gerald's right. While the bus is in the air, Helga reappears, and she is standing where Arnold originally was. When the bus lands and keeps driving, there is one shot where Arnold is missing from the bus, then he reappears, and he and Helga are back in their original places. (01:04:42 - 01:05:22)
Revealing mistake: When Helga tells Arnold that she likes him a lot, as Arnold says, "You do? You did this for me?", his mouth disappears for one frame. This is easier to see in slow motion, but it's still visible at normal speed. (00:57:26)
Other mistake: When Arnold says that he needs a VCR to show the tape, and the camera moves over to Bridget, her appearance is way off: the yellow and gray pieces are missing from her belt, her collar piece is missing, her boots are just dark brown instead of light brown and dark brown, the lines on her suit are not drawn in, and the VCR and zip line device are not in her hands. All this stuff appears in the following shots. (01:07:05)
Continuity mistake: When Bridget uses her zip line, Helga's arms are folded, and Gerald is standing next to her. In the next shot, her arms are down, and Gerald is standing next to Arnold. (01:07:19)
