Star Trek The Motion Picture Bloopers Page
Scaffolding
When Kirk goes out into space, you see a hatch on the Enterprise open. When you see Kirk standing in the hatchway (from the outside of the ship) look at the right hand side of the screen. The scaffolding used to hold up the set is so obvious I can't believe no one noticed it in post-production!

Kirk Switches Helmets
This occurs right after Kirk steps out of the Enterprise into space. When he steps out he is wearing a rectangular helmet. In the next shot he is wearing a round helmet.

Changing Clothes
At the very end of the movie, Spock and McCoy are on the bridge. Spock's coat has an orange band on the sleeve, and McCoy's has a green band on his sleeve. The shot changes, and all of a sudden, Spock has the green band and McCoy has the orange band.

Music Skip
After Chekov comes back to the bridge after his hand gets burned, there is an obvious skip in the music.

Spocks Hair
Throughout the movie, The length of Spocks sideburns keep changing.