Factual error: At the start we are shown Navy Seals approaching a boat under water. Their oxygen system is giving off bubbles. If they were Seals, they would have a system where bubbles would not be given off, to avoid being detected at the surface. (00:05:00)
Revealing mistake: In the scene at the beginning, when Vin Diesel has rescued the professor and has gone into the helicopter to find the pilot dead, he walks back through the door to call the professor, and gets shot. He slides down and you see a bullet hole in the door. Yet before he gets shot, as he starts to walk through the door to call the professor, the bullet hole is already in the door. (00:05:40)
Continuity mistake: At the beginning of the film Vin Diesel is shot, presumably in the chest or torso, but in the shirtless scene during the Girl Scout troop meeting he doesn't have a single scar. (00:06:05 - 00:36:20)
Character mistake: After the Lieutenant is shot and is in hospital, his CO is explaining what the GHOST system is. He explains that the acronym is "Guided High Altitude Scrambling Transmitter" - which would be GHAST. Replacing "altitude" for "orbit", which is probably what was intended, would make a Guided High Orbit Scrambling Transmitter - or the GHOST system henceforth referred to in the film. (00:06:25)
Visible crew/equipment: When Shane is in the car with Zoe, he yells "Go!" and the car takes off past the camera. If you look very carefully as the car moves past the camera, you can see the camera reflected in the car window very clearly. (01:02:05)
Visible crew/equipment: When Shane is talking to Mrs. Plummer before she leaves, throughout the scene the camera man is reflected in her window (he is in black, not to get confused with Shane's white shirt) and as the car drives away the camera is shown backing up in the black door.
Factual error: Shane is a lieutenant and the Principal is Petty Officer first class. Shane states the Principal is the senior officer. This is wrong as the Petty Officer is a Non Commissioned Officer, while Shane is a commissioned officer. Shane outranks the Principal.
Continuity mistake: When Vin Diesel gets hit by the sprinklers at the beginning when he gets in the house his pants are completely dry.
Revealing mistake: When Shane is fighting the two ninjas, they are armed with expandable batons, which are tubular hardened steel. During the fight sequences you can see the plastic batons bending repeatedly.
Continuity mistake: After Shane Wolfe and Mrs. Plummer retrieve GHOST and then see the children, as Principle Fletcher jumps on Mr. Chun, you can see all of the police officers reach for their guns and Shane bend slightly to seem like he is reaching for something. As the shot changes to behind Shane and the police officers, Shane is holding the gun that Mr. Chun had and the officers to his left are reaching for their guns, but the officers to his right still have their hands up. When the shot changes to show their faces, Shane is standing there empty handed and all of the officers now have their guns in their hands.
Continuity mistake: When the baby throws up on the babysitter, the vomit mainly hits her shoulder with very little on her face. During the next shot as the babysitter starts to clean up, the main portion of the vomit appears to have hit her face instead.
Continuity mistake: Peter is first seen when Ms. Plumber comes to the door, he has one of his shoes on. When Ms. Plumber closes the door, Peter has both shoes off.
Continuity mistake: When Shane and the kids are in the arcade/pizza place, from Shane's view you see a woman in a print dress tugging on a kid. Change to see Shane's face and she is dragging the kid away behind him. Change back to Shane's view she is still trying to get the kid away from the game.
Continuity mistake: When the Captain and Mrs. Plummer return in the black Ford Crown Victoria, the Ford emblem is missing on the grill. When the Crown Vic crashes into the car dealership later on, the Ford emblem is intact.
Continuity mistake: When Vin Diesel is side-skidding the minivan into a parking spot, there is a close-up of the rear tire skidding to the curb and a row of grass, but in the wide shot there is no grass next to the curb.
Continuity mistake: When Vin Diesel does the panda dance to get to the safe, all the stuff comes down, including pins and gates. In the next shot, you see all the gates and pins going back up, except for one. When the camera changes angle, the pin is up, even though there isn't such a big time difference to have the pin go up, and the viewer not to see.
Continuity mistake: When Shane is trying to stop Helga from leaving he is on the phone with Julie Plummer. He says goodbye to her and jumps in front of the door while hanging up the phone, the camera angle changes to Helga then back to Shane and he is no longer holding the phone.
Other mistake: When Shane crashes the car through a fence (or wall), it appears that the car sustained absolutely no damage on the front (or anywhere else as far as I could tell). Later, there seemed to be very little damage to the car as a result of a chase scene where the car hit things, just a relatively minor scrape or two.
Continuity mistake: Near the end of the movie when Captain Fawcett is pointing the gun at Wolfe and Mrs. Plummer, the safe's door springs back to knock him out. At the moment the safe's door seems to be about 80 degrees open, yet when Shane opens the safe for himself, it appears to be about 30 degrees open.
Continuity mistake: The wrestling coach is seated behind the principal at the Sound of Music play finale. He is wearing a brown suit, has a cervical collar from his injury, and is behind a woman in a green sweater. A few minutes later, when the principal is talking to Vin Diesel about the wrestling coach finding a new calling, we see the coach on stage dressed as a nun, singing and taking bows. The camera then takes one last shot of the audience standing up and we see the very tall coach figure in the rear of the crowd, minus his collar. Brad Garrett's figure is an unmistakable one due to his size.
Answer: No, it is just his sock.
It's a cast with a sock over it. Since the movie is shot out of sequence, possibly he might have hurt it during production.