The Single Biggest Mistake Each Star Trek Movie Has Made
10. Star Trek IV: The Voyage Home - Pollute The Past Much, Pavel?
The Mistake: Chekov reveals WAY too much information during his interrogation.
Why?: What was Pavel thinking? He is captured by the guards aboard the contemporary USS Enterprise after he and Uhura steal the fuel they need for the Bird-of-Prey. Sitting in front of him, the interrogator grills him for information. Chekov then gives him his name, rank, and service number. Repeat: Chekov tells the American Navy about the existence of Starfleet.
This could have an enormous impact on the timeline. Frankly, Chekov is blessed as the interrogator doesn't believe a word of what he says, yet he leaves his technology behind to confirm his story. A Klingon phaser and communicator remain in a highly guarded military location and no one thinks to return for them? There was an entire episode of Enterprise about this very topic!
Finally, though this is of course accidental on Chekov's part, he requires emergency brain surgery at a local hospital. McCoy is able to save him easily, though the surgeons in place witness the use of highly advanced machinery in the process of this, not to mention Kirk casually firing a phaser at the door handle.
The Voyage Home is easily the most fun Star Trek film but it would have been a nightmare for Temporal Investigations.