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)
Continuity mistake: When Michael breaks the safety glass for the axe, some of the glass remains intact. When it cuts the glass is gone. (01:01:25)
Continuity mistake: When Norma places the case on the table, Ana says she shall go check on Luda while placing a box horizontally in the case. When it cuts the box is lying vertically. (01:08:00)
Continuity mistake: After Norma has been killed and Ana says someone should say something, Kenneth moves forth while saying "There is nothing to say," and Nicole comes out from behind the counter and walks a few feet. When it cuts she is walking out from behind the counter again. (01:12:50)
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 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)
Factual error: When they are eating, there's a box of chicken labelled PFK (Poulet Frit a la Kentucky), the French equivalent of KFC. This film takes place in the USA, but was filmed in Canada where most products are labelled in English and French.
Continuity mistake: When the gang of survivors goes to Andy's gun shop, one of them shoots a zombie and he proceeds to fall on his knees. The scene cuts to the gang and then to the charging zombies and the shot zombie falls again.
Continuity mistake: In the scene where the remaining survivors are on Steve's boat and Michael stays because he's bitten, the sun changes positions. It goes from being morning to afternoon.
Continuity mistake: At the birth of the zombie baby and the shoot-out, the pregnant girl gets shot through the left eye blowing her eye socket apart. Afterwards, the main blonde girl surveys the area and now the pregnant lady's left eye is undamaged.
Other mistake: Near the beginning, Ana speeds away from her home down winding streets in the suburbs. In an aerial views of the houses, look closely at the row of homes at the bottom of the screen. None of the homes are shown with driveways or sidewalks. Essentially, there is no way getting to these homes. This is revealed in the commentary by the director who realized the mistake, but didn't care.
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).
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.
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.
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: Since it's not addressed in the script or the film, any answer would be speculation. One could be that Luis is just a generally laid back person, and the appearance of a young child he knows is not enough to ruffle his feathers, even if she isn't supposed to be there. He also just woke up, so he could be a bit groggy and not fully processing the situation as quickly as he might normally. More likely, it's simply the way the scene was filmed; the director and actor didn't think it important to show Luis "bothered" by the girl's presence (prior to noticing the blood, of course).