Continuity mistake: When Ana is driving home we see an above shot of her car as she approaches Vivian. When it cuts to Ana stopping right next to Vivian you can see there a far more cars in the background than it was in the previous shots. (00:02:35)
Continuity mistake: In the beginning when Ana is standing beside Dr. Dandewar who is speaking on the phone we see her looking away from him. When it cuts to him hanging up the phone she looks at him and he has moved a feet away from the wall. (00:00:50)
Continuity mistake: When Andre is walking alone and gets to one of the mall's entrances he wipes his forehead. When it cuts he is wiping his eye. (00:20:10)
Plot hole: When Ana is driving away from her home, she stops on the road and a guy opens her door, scaring her into turning her car toward the curb, plowing through a guardrail. She couldn't have picked up a lot of speed, and those guardrails are supposed to always be reinforced on highways/interstates. At that angle she would have had to knock out at least 4 reinforcing beams (they use 'I' beams to reinforce them, and are intended to stop vehicles from passing through them, or slow them down immensely).
Continuity mistake: When they crash their truck Steve jumps out of it. The truck is quite dusty and Steve's hand slides on the side of the truck leaving a mark in the dust. Later when Kenneth jumps out of the truck you can see the side has been wiped clean, removing not only Steve's mark, but a whole area of dust. (01:33:50 - 01:34:20)
Continuity mistake: When they are going to rescue Nicole from Andy's store, Michael shoots a zombie in the head and blood splatters on the side of the truck. The amount of blood on the side differs between the shots. (01:24:25)
Continuity mistake: When they are backing up the truck, it hits the fence on the left side bending almost the whole fence. When it cuts the fence is suddenly intact. (00:41:05)
Continuity mistake: The E.M.T.'s legs in back of ambulance when first shown are hanging over the red line on bumper. When they cut to farther shot, The legs are now pulled up more. (00:02:05)
Continuity mistake: In the beginning when Dr. Dandewar hangs up the phone, there is an orderly pushing the cleaning cart in the distance behind Ana. When it cuts to the close-up, the orderly disappears with the cart. (00:00:45)
Continuity mistake: When Ana and Kenneth are walking through the tunnel, as Andre fires at them, Kenneth is standing in the sunlight shining in to the tunnel. When it cuts to Kenneth and Ana bending down the sun isn't shining inside the tunnel anymore. (00:13:55)
Continuity mistake: After Andy shoots the zombie that looks like Jay Leno, there are shots of the characters looking through binoculars. Tucker's and Steve's binoculars still have the lens caps on. (00:58:05)
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.
Continuity mistake: At the very beginning, when Luis attacks Ana, she escapes into the bathroom and falls over into the tub. The rug on the floor is folded back in the shot from the top, but it is flat when Ana steps out of the tub.
Continuity mistake: In the opening scene after Anna's neighbor is hit by the ambulance, his body is visible lying in the street. However, when Anna is driving away you cab see the street through her car's rear window and the neighbor's body is nowhere to be seen.
Continuity mistake: When the group goes to rescue Nicole from Andy's shop it it is evening, when they return and must evacuate the mall it's night, but soon as they clear the horde in the parking lot it becomes early morning.
Other mistake: When Kenneth and Andy exchange via white board about Fort Pastor, you can see a guy chilling on the roof of a building nearby in the background just over Andy's head when he ask " so what's the bad news." He is not trying to get the attention of either Kenneth or Andy. (00:47:28)
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.
Continuity mistake: About 3/4 of the way through, CJ is on the roof of the gun shop and blows up a tank of propane to clear the zombies so everyone can escape. The truck that was blocking the door was their only way in and out of the shop. On their way into the gun shop you see two zombies follow them through the truck to the shop's entrance, but when he blasts the propane, somehow I guess they get cooked. My nitpick is that when CJ goes the same path as the others, there is a dead zombie blocking his exit through the truck's door. If the others already went out that same door they would have had to move that burned zombie to get through the truck.
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