Okay, bit of a spoiler alert here, so if you haven't seen the movie and want to, look away.
For anyone who read the book, here's a rather large flaw I saw that I can't get over. I'd love to know how they explained it in the book.
We see the reason they have to leave early from their mission is because their MAV is about to tip over (beyond 13 degrees is fatal they say). So why would they preposition the MAV for the next mission so early, exposing that ship to the known winds/sandstorms of Mars for so long? That seemed like a rather large logic flaw.