Continuity mistake: At the start, while the neighbourhood 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: 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: When Helga is talking to her Arnold shrine, the candles around her constantly change position between shots. (00:22:23)
Audio problem: When Helga is talking to her Arnold shrine and the pigeon defecates on her, as she says, "Perfect," her lips don't match what she says. (00:22:44)
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: When Arnold and Gerald walk out of Mr. Bailey's room, there is a door with the words, "Municipal Services" on it, which are black. When Mr. Bailey goes to talk to them, they change to yellow. (00:26:20)
Continuity mistake: When Arnold and Gerald walk out of Mr. Bailey's room, there are blinds on Mr. Bailey's window. A few seconds later, as Mr. Bailey is talking to them, they disappear. (00:26:23)
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 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)
Revealing mistake: When the head of security carries Arnold and Gerald into Scheck's office, as the camera zooms in on Scheck, one of his buttons disappears twice in two frames, and his other button completely disappears mid-shot. (00:30:18)
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)
Other mistake: When Bridget tells Arnold and Gerald, "If you're gonna try and save the neighbourhood, you're gonna need some equipment," Arnold's right eye duplicates. (00:35:55)
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 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. Later on, when she shows them the watch, the binoculars are brown. (00:36:19)
Revealing mistake: When Arnold asks Bridget how much the equipment costs, as she says, "It's on the house," her eyebrows disappear for one frame. (00:36:35)
Continuity mistake: When Arnold and Gerald walk to the bus stop in their suits, there is nothing next to the bus stop sign. When the bus leaves and Helga starts talking to herself, a sewer drain appears next to it for Brainy to come out of seconds later. (00:37:47)
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)
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)
