Continuity mistake: When SpongeBob says he should sacrifice himself, the squidosaurs Rex has Mr. Krabs in his hands, but when SpongeBob walks to the counter, Mr. Krabs is right next to Squidward. When the camera zooms in on a group of citizens and Squidward, Mr. Krabs is gone. Then, another shot of the squidosaurs Rex, Mr. Krabs is in his hands again.
Continuity mistake: When Squidward scares the people off, some of the burgers they drop disappear and change position.
Continuity mistake: When SpongeBob and his crew are stranded on pelican island, Squidward releases a puddle of ink on the ground because he is nervous. When SpongeBob dabs the pen in the ink, in the next shot the ink is completely gone.
Continuity mistake: During the sequence of plankton about to steal the krabby patty formula, there's a ship in a bottle on Mr. Krabs desk. When SpongeBob arrives with a vacuum, it's gone.
Continuity mistake: Spongebob and Plankton are stuck between a large crowd. Some are facing their backs to the Chum Bucket, the others are facing their backs to the Krusty Krab. Later, the group facing their backs to the Krusty Krab disappears.
Continuity mistake: When Patrick is holding SpongeBob, he yells to the others that he has him. Two shots later, the background of SpongeBob, Patrick and plankton has changed - specifically, the pineapple house disappears.
Answer: The point of a trailer for a movie is to get bums on seats in cinemas so the film-makers have to make a film look really good and generate a "I really have to see this" vibe. Because virtually all professional films shoot far more material than they need, some of it will end up in the trailer as exclusive content and some will end up in the special features section of the DVD (bloopers, deleted scenes etc).
Neil Jones