Continuity: When he's walking in the subway, Rob's shirt has a dark triangle of sweat (and dirt) on the back. In one clip the triangle is barely visible and the shirt looks a lot cleaner than before. In the next clip, the triangle is very clear again.

Factual error: After Rob saves Beth from her apartment, they work their way up the leaning building. When they reach the top and they are walking up the stairs to the roof, the stairs are perfectly horizontal even though the building is crooked.

Factual error: I just watched this film on DVD. I know this is a rule of the genre (see 28 Days/Weeks and Die Hard movies) but I find Beth's remarkable recovery from being speared with an iron bar - and stuck there for hours losing blood circulation and muscle cramping - to running around in Central Park helping Rob wholly unbelievable. Maybe she could limp down her building stairs, but whatever physical efforts she does afterwards are even more impossible than those of the monster.