Revealing mistake: While Dr. Wentworth is driving his car trying to shut off the loud music on the radio, the dashboard shows that the car's gearshift indicator is not in "D" for "Drive."
Revealing mistake: The shot of the two nuclear missiles being launched is used twice, just reversed. They seem to have changed the saturation slightly in one of them to make it look different, but there are identical clouds at the top left/top right in both shots, ruining the trick.
Revealing mistake: Just after the helicopters crash during the chase with the crop duster keep a close eye and one of the helos is still in the shots flying.
Revealing mistake: During the first rollercoaster accident, the bodies in the cars are obviously dummies. (00:13:35)
Revealing mistake: In the beginning of the movie when Orca is about to hit the great white shark out of the water the stick that holds the "head" of the great white is visible as it swims by.
Revealing mistake: While the plane is almost upside down, papers and stuff fall inside, but one can notice that it's the actors throwing the papers, not gravity making them fall.
Revealing mistake: In the beginning when Shockley pulls up and the Jack Daniels falls out you see it hit a wet mark from the previous take.
Revealing mistake: When the bat flies into the bathroom and attacks Suzy you can see a string attached to the fake bat. (01:19:16)
Revealing mistake: At the beginning, when Gail is attacked by a moray eel - we don't see it, but that's the assumption - and she is banged against the sunken wreck of a ship, and then, when she is released, her mouthpiece is not in her mouth and she tries to swim to the top as fast as she can - all of this is done by a stunt double, understandably, we can see this by her hair, which is much thinner and straighter than Jacqueline Bissett's hair.
Revealing mistake: After Ethel has died and Brenda is crying over her, there is a slight twitch in Ethel's eyes.