Corrected entry: During the initial car chase scene, the speedometer of the stolen car is shown a couple of times. You can see that the speedometer is in kilometers per hour, not miles per hour. This is because most of the movie was actually filmed at a studio in Rome, Italy, so the cars they used were built with European specs.
Corrected entry: In the scene where the control room operators are looking through the tunnel security camera, you see a crack in the picture, apparently illustrating that the camera lens is damaged. The cracks are perfectly in focus, but a camera lens can't focus on itself, let alone focus on two different distances at the same time. If the lens were really cracked, all you'd see is a blurry and/or fragmented image.
Corrected entry: Right after Stallone goes through the wormhole to find Roy, you see all the survivors talking outside it. Madeline says to George something like, "Here, take this walkie talkie." George refuses and Madeline says, "Jesus Christ..." If you look closely, her lips aren't moving.
Corrected entry: In the scene where George(the security guard) is stuck under the truck, the rest of the people lift him out from underneath. When they are moving him away, his head is bobbling around. Later on, they can't move him because his neck is broken. If they moved his head at all, he would have died.
Corrected entry: When George the cop finds Roy, he asks if his cell phone works, which Roy responds that it doesn't. Then when you see him climbing through the wormhole, he's talking to somebody on his cell phone.
Corrected entry: In the scene when Stallone uses the plastic explosives to block the water from entering the tunnel which contains all the survivors, he climbs up on some wreckage and shoves the explosive charge into a big pile of mud. When he pulls his hand out, it's perfectly clean. One would think he would have at least some mud left on his arm, seeing how far he shoved it in.