The Single Biggest Mistake Each Star Trek Movie Has Made

The Mistake: Sending your Captain and First Officer down to an unsecured location.

Why?: Although this one is a bit self-explanatory, it's the disastrous fallout that comes later that reinforces the issue. Captain Clark Terrell and Commander Chekov beam down to what they believe is Ceti Alpha VI to investigate an unidentified lifeform. It's really quite difficult to understand why they, out of everyone in the crew, were the ones who had to go.

Starfleet has a strict set of rules in place when it comes to the captain of a starship going on an away mission. The events of this movie can only have served to validate this. Terrell beams to an unknown situation and is quickly captured, brainwashed and enslaved by Khan. While the loss of a captain is bad enough, it then results in the loss of a starship.

The USS Reliant is overrun by Khan's forces, allowing them to finally escape from the surface. Meanwhile, the entire crew of the Reliant are stranded. The ship is later destroyed in battle. While it was necessary for Khan to gain control for the sake of the plot, literally any other crewman should have joined Chekov in that transporter room.

And all of this is just quietly ignoring the continuity error about Khan recognizing Chekov, of course...