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: In the book, he's stranded on sol 6, and leaves on sol 549, making it 543 sols (554 days). In the movie, he's stranded on Sol 18 and leaves on sol 561, making it 542 sols.