Blooper

During the martian attack on the jail, "Akooshay" is seen being grabbed from behind and stabbed in a medium shot. In the subsequent wide shot, this can be seen happening again in the background.

The gravity on Mars is a fraction of that on Earth, yet none of the character's actions or movements suggest that.

Skill saw blades can inflict severe lacerations when thrown; however they are not sturdy enough, nor could they be thrown hard enough, to amputate limbs or heads as shown in the film.

Mars has a very thin atmosphere, no magnetic field and its daily temperatures are well below zero. Unless the planet was changed using technologies not shown in the film, there's no way that the characters would have been able to walk around without spacesuits and breathable oxygen.

This was actually mentioned in passing when the cops exit the train. They talk about how they had to use full masks just a few years ago; it is also quite clear from context that the planet was/is being terraformed.

When Ballard breaks free from Williams' grip, the knife at her throat can clearly be seen to bend with her neck rather than slice a gaping wound in it.

During the battle when the survivors are waiting for the train to arrive the first time, a sword makes the noise of hitting metal when colliding with a gun, but it bends significantly and is obviously made of rubber.

During a shot out the front of the police station, a crew member is visible loitering in the shadows while the leader of the Ghosts yodels in the center of the street.

When Ballard and Williams are attempting to escape in the train, several of the possessed people jump on the back and climb onto the train car, and start banging on a hatch. The train has three sections separated by open air (or not so much air, on Mars). Even though Ballard and Williams are in the engine compartment at the front, they clearly hear the metal clanging coming from the last car, which is not directly connected to their own.