Revealing mistake: As Ana is driving away from her home, when we see her car driving from a shot high above, we see a car crashing in to a gas station making a huge explosion. However the trees a few feet away from the explosion don't move. The shockwave would have made them react at least a bit. (00:09:20)
Revealing mistake: When they use a gas cylinder and shoot it with a shotgun to wipe out a load of zombies, the resulting explosion as seen from above the trucks is way too big to have come from that small cylinder.
Revealing mistake: When Ana and Luis shower together at the beginning of the movie, Ana's hair is very long, almost down to her waist. This is obviously a body double as the real Ana's hair is much shorter.
Revealing mistake: When the very pregnant Luda washes blood off of herself, she leans over the sink. The actress' very flat tummy is visible underneath her red top.
Answer: It was actually explained in both versions that the zombies went to the mall because it was a place of familiarity, a place that they could remember even if they didn't know why.
Tobin OReilly