In film making, there is a person in charge of continuity--ensuring things on the set appear exactly the same in consecutive scenes. Usually, pictures are taken of the set and used as a reference, but occasionally one thing or another is overlooked and a discrepancy gets past the editors and director, to become part of the finished product.
I've personally spotted a few errors in movies such as the backwards running scene in Anaconda, in which a boat floats past a waterfall that is flowing upwards instead of down. Then there was the scene from Forrest Gump that featured an iron on an ironing board in the background going from vertical to horizontal and back to vertical in the course of a few moments. Also, in a scene from "Raiders of the Lost Ark" a reflection of the cobra "dangerously" close to Indie's face is seen in a protective glass pane.
Anyway, culled from the internet, here are some other instances where the continuity director was asleep at the wheel:
1. In the Lord of the Rings, a car can be seen traveling down a dusty road off in the distance.
2. In the Battle of Carthage scene of "Gladiator," a gas tank attached to a tossed-over chariot can be spotted briefly by the sharp-eyed viewer.
3. In the Wizard of Oz, the tin man is first found in a thoroughly rusted condition--even though tin does not rust. Also, during the musical duet between Dorothy and the scarecrow, the length of Dorothy's pigtails changes several times within a few minutes.
4. In a scene from "Pulp Fiction", bullet holes are seen in a wall prior to the firing of the bullets that made them.
5. Among other goofs in "Commando", the wrecked drivers side of Sully's yellow Porsche--visible when Matrix flips the car back onto its wheels, is transformed into pristine condition in the very next shot.
6. In North By Northwest, a kid in the background is seen putting his fingers in his ears just before a gun he can't see is fired--probably due to his having been through several takes of that loud scene.
7. In "Twister", a large crack in the windshield of a car disappears and reappears within moments.
8. While not exactly a continuity error, in "Forrest Gump" Forrest is portrayed as reaping the rewards of fortuitously investing in Apple in 1975--5 years before the company went public.
