Plot hole: At the end of the movie where David and his friends are attempting to break into Steve's house, David has the door code and attempts to gain access once by using it. Steve then tells his wife and the kids to go hide upstairs and turn off all the lights so nobody will know where they are. Then Steve and his wife go to another part of the house, away from the front door. Then, we hear David and his friends using axes and a big log to try and break down the door, "before* Steve secures the door with household appliances later on. Why wouldn't David have just kept trying the code?
Suggested correction: The reason David can not keep trying the code, is because the door has a lock on the top, and one at the bottom (middle regular lock). That middle lock is able to respond to the key/pass-code system, but the top one is not. In order to get in the house, David would have to be able to enter the pin (which, yes he knows) but with the top lock still locked from the inside he can not get in. I am assuming there is a key slot on the outside where he would need a key, and also I'm sure the family doesn't bother locking the top half of the door as no one else knows the pin (so they thought). And I'm guessing that they never locked it during the day either, and only at night.
Plot hole: If the dinosaurs are so dangerous, why are they only kept behind plastic sheeting at the back of the truck, rather than in cages? There are computers in the truck, so obviously people must go in there, and it wouldn't be very safe to use computers if dinosaurs were roaming free in the trucks.
Plot hole: When Peyton/Darkman is showing Bridget how the liquid skin works, he drips a small amount onto his burnt finger, and it solidifies. However, when he drops it onto his finger, it magically shapes itself into a replica of a perfect, undamaged finger. This makes no sense, as it is simply a liquid and is not being molded by any forces. Hence, it should not look like a perfect finger.
Plot hole: When Seth's piledriver gives out during the climactic fight, there are many vampires in the background politely keeping their distance while he tries to restart it. Similarly, they ignore him while both his hands are occupied with Sex Machine's whip, even though vampires are running around behind him to no apparent goal. (01:31:35)
Plot hole: Given the magnitude of the crimes that occur at the house at the end, the police can be seen in the last shot dusting Dewey's truck. If the police are so careful as to dust for prints when the entire rest of the crime scene is heavily contaminated by everyone from the EMTs, Gale doing her news, police, etc. makes what they are doing seem silly. (01:43:25)
Plot hole: When Earl, Grady, Burt and Kate are running from the truck that Burt shot, they go towards the cantina, which has a couple of smaller blue and white structures beside it. In a wide shot as they're running, we see the 1st shed at the side of the frame. A moment later Earl sees several walking Graboids in front of the 2nd shed. The beasts were in plain sight, yet nobody noticed until they were almost on top of them. (01:14:50)