Character mistake: In a scene when Watney is recording his video log, he imagines what would happen if his pressurized shelter suddenly would break. He concludes that he would implode, but the correct wording would be explode.
Character mistake: The flags (both USA and China) on Iris 2 when hung vertically, were hung incorrectly, and appeared as such on the cover of the "Times." They should have been "reversed" so that canton is still in the upper left hand corner. (01:30:35)
Character mistake: As Teddy is announcing the bad news, he calls Mark Watney 'Mark Watton'. (00:09:48)
Answer: Part of his job, aside from also being a mechanical engineer, was to use soil taken from Earth to Mars, mix it with Martian soil then grow seeds in it to see how Martian soil is for growing crops. This would be preparing for a longer term mission where growing full crops to feed the crew would be part of the mission.