(at around 57 mins) In the hospital scene when the bath is overflowing, there is a massive amount of water pouring out of the bath, but only a small stream going into it from the tap. It would be impossible for more water to be coming out of the bath than going in.
(at around 4 mins) When one of the race cars makes a pit stop near the beginning, and then takes off and hits the puddle of oil. The sound of the "squealing" tires is clearly the sound of a dolphin chirping and not tires.
(at around 2 mins) During Nick's premonition, when the cowboy sits in front of Lori, something happens on the track which causes the audience to stand up and gasp. After Nick's premonition, the cowboy sits in front of Lori, but the audience remains seated.
(at around 1h 9 mins) Once Nick awakes from the mall premonition, he rushes through the mall to find the construction site. In one shot, you see him pushing through some people. For a brief second, you can also see Lori and Janet chasing him. They are supposed to be in the movie theater at this moment. (This shot was in the alternate ending where Nick finds Lori and Janet before the movie starts and warns them of their impending deaths.)
(at around 31 mins) When Hunt and Janet pay a visit to Nick and Lori to discuss about the possibility to exist a chain of deaths or a specific order, Janet leaves and Hunt follows her. Immediately after he gets up the couch, his shirt allows some belt to be visible in his back. He turns around to say something, and when he leaves, his shirt is actually OK.
(at around 43 mins) During the car wash scene Janet is seen having trouble with the moon roof on her vehicle. When she drives up to the car wash you can seen the top of the vehicle and the moon roof is clearly closed. Minutes later Janet is seen closing the moon roof.
(at around 39 mins) Nick says he can't be sure who is next to die, Hunt or Janet, because in his premonition, they were both killed in the same collapse. However, he is confident that George dies before Lori even though in his premonition, they were both killed by the same explosion.
(at around 1h 8 mins) During the escalator scene, Lori's right foot was crushed by the escalator's conveyor belt. This makes her spit out blood, and that is not supposed to happen. Since only her limbs were hurt or injured, she wouldn't spit out blood. We only spit out blood if our head, chest, or abdomen are injured or hurt.
(at around 1h 13 mins) The sprinkler system that saves the movie theater would not work. Water cannot put out a chemical fire, you would need a foam system, and not only would it not put it out but it would spread the fire and make it wider.
(at around 1h 11 mins) A nail gun cannot shoot without the trigger being pulled back, and the front end (where the nail shoots out) being pushed into the wood; furthermore, electric nail guns have to be within certain angles in order to shoot. It would be impossible for the gun to shoot like it did in the movie.
(at around 57 mins) When the old patient at the hospital tells the Asian medical worker that he killed a lot of them in Korea, the Asian medical worker replies that he is Chinese, implying that the Chinese did not participate in the Korean War. However, China did participate in the Korean War and many Chinese soldiers died there.
(at around 1h 12 mins) Sprinklers work by holding back water with a heat sensitive liquid filled glass bulb or soldered struts and the heat from a fire burst the bulb/melts the solder allowing water out. In the film the sprinkler head is open, with no bulb or struts so the system must be drained down and the main valve shut (or water would be gushing out all the time).
In which case you could fire a flamethrower at it, never mind a bit of burning wood, and it wouldn't activate.
(at around 49 mins) A pump cannot suck at 160 psi a mere three meters under water. The total pressure of water and atmosphere at that depth is 20 psi. Since 0 psi is the lowest achievable pressure, Hunt cannot be subjected to more than 20 psi. A mere 20 psi pressure difference would not cause the spectacular effect seen in the movie. It would at most give him an embarrassing hickey (and drown him).
(at around 1h 8 mins) In the movie house scene after the fires and explosions happen we are shown a large propane tank indoors with fire around it. This size tank is only mounted outdoors where there is plenty of ventilation. Never indoors.
(at around 45 mins) In the car wash scene, Janet is shown trying to call for help but is unable to since there is "no signal", but when the phone is shown, 'Offline' can clearly be seen on the upper corner of the screen. This is only displayed if the offline mode (used while traveling on airplanes) has been manually enabled.
(at around 1h 13 mins) Right as Nick sets off the sprinklers with the long board on fire, you can see him twitch and slide around his other arm slightly, which is supposed to be nailed to the wall and therefore immobile.