Revealing mistake: In the beginning of the movie, when the rescue group arrived on Isla Sorna, the actress grabs the Nikon F5 of the photographer and makes her way to the nest of the Stegosaurus. After several shots of the small Stegosaurus there is no more film left in the cassette, so the camera begins to rewind it automatically. The thing is, you never see the rewind knob moving although you hear the camera's motor rewinding (The Nikon F5 has a manual rewind knob to be able to rewind the film when the batteries are dead). There was obviously no film at all in the camera. (00:26:00)
Revealing mistake: When the stegosaurus begin to move around a lot, you can see the one that is charging at her steps in the creek water, but there is no splash or any ripples. (00:26:20)
Revealing mistake: The window in the door of the trailer that got pushed over the cliff is quite obviously made of a plastic film. You can see that it is wobbling when they bang the door. (00:57:28)
Revealing mistake: When the T-rex escapes into San Diego, it enters someone's backyard to drink from their pool. When the Rex plunges its face into the water, the water barely ripples and its bottom jaw is as clear as day below the surface of the water. If a beast of that size were to dip its head in the water, no matter how gently, it would've created a ripple that would distort the surface of the water quite significantly. (01:45:20)
Revealing mistake: When Sarah is making her way to the baby Stego, just before she gets on her belly and crawls closer to it, you can see the kneepads she has on.
Revealing mistake: When Kelly does her acrobatics and kicks the raptor out through the window it is possible to see that the raptor is pulled through the window as it "flies" upward and scrapes against the top of the window frame and then straight down. As the raptor falls down 2 wires can be seen reflecting light outside of the window, one even falls down after the raptor.
Revealing mistake: When the dinosaurs are released from their cages and are running around the camp, a jeep is launched into the air and lands almost on top of the 2 hunters who leap from the tree. When the jeep falls forward, and down out of the tree the bottom of the tree moves and shakes in a way it shouldn't if it was truly made of wood.
Revealing mistake: When Stark is being killed by the pack of Compys, the "blood" that flows down the stream is obviously a computer generated effect. It looks nothing like liquid at all.
Revealing mistake: As the T-Rex rages through the street, some cars crash, and one reversing car dramatically flips. Two problems though - firstly, it's reversing quickly enough to mean it must have travelled past the dinosaur to build up that much speed, so the driver would have just driven forwards away from the T-Rex instead. Plus there's nothing which would realistically be in the road to make the car flip over like that, it's just a street with cars stopped on it.
Revealing mistake: The woman who screams and reverses away from the T-Rex while staring at it is suddenly replaced by a male stunt driver looking over his shoulder in the shot from the side.
Revealing mistake: When doing the scan on the T-Rex leg it's clear the Doppler isn't touching or even facing the dinosaur, but an image is shown on the screen.
Answer: Just after she makes the statement, Ian sees the T-Rex outside of the van window and realises that it is going to attack them regardless of the fact that they've returned the baby Rex.