Continuity mistake: When the gas jeeps stop for the first time they are in sunlight away from trees. At the end of the scene they are in the shade by some trees, despite them not moving in the scene. If you watch closely when Hammond tells them to stop they are in a massive patch of sunlight as shown with Dr Grant standing up. (00:21:05)
Continuity mistake: When the T-Rex turns the upside-down car around 180°, the car's tail lights move from right to left. When Alan and Lex try to move out of the way, the tail lights are moving from right to left again.
Continuity mistake: When the Dilophosaur spits on Nedry the second time, it is only on his forehead and eyes. In the next shot, a second later, it is all over his face and cheeks. It was too soon for the rain to have washed it down that far.
Character mistake: When Genarro is explaining to Hammond that he is at the park to report Hammond's progress to the investors, he says "In 48 hours if they're not impressed, I'm not impressed. We'll shut you down, John." He should have said "If I'm not impressed, they're not impressed" since the investors would be shutting down the park on his advice, not the other way around.
Continuity mistake: In the tree, before Lex tries to feed the dinosaur there are no branches or leaves in front of her. She leans forward and a shot later a branch filled with leaves has magically appeared by her waist in a mere nanosecond. This keeps changing back and forth depending on the angle.
Continuity mistake: After Hammond says "Welcome to Jurassic Park" Grant is facing the jeeps, 3 meters away, with Ellie 5 meters to his right and Hammond 3 meters to his left. Grant then turns his waist around to the left, though his legs still face the jeeps. In the wide angle everyone has changed positions drastically: Grant's legs have magically turned 180° and Hammond is not only opposite Ellie but very close to her.
Continuity mistake: Ian gets out of the car to distract the dinosaur. Inside the car, he's nice and dry, but the second he gets out of the car, he looks like he's stood in a bathroom shower for ten minutes.
Continuity mistake: When Nedry's car is stuck in the mud, he goes out front to grab his winch and pulls out a fair bit of wire. In the next shot, we see that he has a lot less wire than before, even though we never heard the winch retract.