Other mistake: When devising the plan to retrieve Watney, it is mentioned that the Hermes crew have had to lash together all the webbing on board to make the longest possible tether. When this tether is used, there is no evidence of any lashing together or other extensions or modifications to lengthen it. The tether is one continuous length and is stored on a reel that was designed for the length of tether gathered on it.
Other mistake: When Watney is in zero-g there are loose bolts floating around him. There is no way he would have left loose pieces in the MAV's cabin. Things left unsecured in microgravity tend to float around and damage delicate electronics. If something is not being used it's lashed down or secured in a cabinet. He has one chance to to get rescued and he wouldn't risk dying after over a year of surviving on Mars on something so foolish as unsecured equipment in the MAV cabin. He would have cleaned out every last bit of loose gear before launch.
Add timeGrumpy Scot
Other mistake: During the storm scene in the beginning of the movie, the astronauts faces inside the helmets are brightly lit, meaning there's a light source pointed directly in their face. That's something that would render them mostly blind and unable to see and appears to be nothing but a dramatic effect for the camera.
Other mistake: Rich Purnell explains his plan to redirect the Hermes to Mars in order to rescue Watney, positioning people to represent planets and using a stapler to show the trajectory of the vessel. He is talking to experienced, qualified engineers and technologists working at a very high level on the space programme. They don't need drama school play acting to be understand things like this. He could have explained his plan in the most complex and abstruse terms and they would have been way ahead of him.