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)
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.
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)
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)
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)
Continuity mistake: When Bridget shows Arnold and Gerald the equipment, there are purple binoculars on the table. Later on, when she shows them the watch, the binoculars are brown. (00:36:19)
Continuity mistake: When Bridget is presenting the equipment to Arnold and Gerald, there is a close-up of some walkie-talkies, but they are never visible on the table in any other shot. (00:36:16)
Continuity mistake: When Bridget shows Arnold and Gerald the equipment, there are purple binoculars on the table. In the next shot of the table, they disappear. (00:35:58)
Continuity mistake: When Arnold and Gerald are standing in front of the City Morgue, the design of the doors changes between shots: handles appear on the panels, extra panels appear, and the top panels become square-shaped. (00:26:42)
Continuity mistake: When Arnold and Gerald are standing in front of the City Morgue, there is a lantern next to the doors. In the next shot, it moves further down. (00:26:39)
Continuity mistake: When the head of security carries Arnold and Gerald into Scheck's office, there is nothing on Scheck's desk. Later on, Scheck goes to his desk and picks up a glass full of cherries, which wasn't there before. (00:31:36)
Continuity mistake: When Scheck closes the doors to his office and starts talking to Arnold and Gerald, there are no locks on the doors. Then, when Arnold says, "I'm Arnold, and this is Gerald," a lock appears under the left door handle, then disappears. (00:31:41)
