When Quinn runs away from the robbery, the number of people in the streets around him change frequently between shots. This is most noticeable just after he loses the bag containing the loot.
Shepard's sub-machine gun changes type during the raid on the nest. At the bottom of the ladder he's carrying a Sterling with a horizontal curved magazine. In the next shot at the top of the nest he's clearly carrying a late model M3 "grease gun" with a straight vertical magazine.
The man wielding a knife has no blood on his shirt, then does.
During the police shootout with the serpent, the monster lands next to a cop in a basket dangling over the side. In close-ups, the cop is clean-shaven with no mustache. But, the cop the creature grabs and throws to his death has a thick bushy mustache. Clearly, there were no clean-shaven stunt men available.
When Shepard visits the museum to ask about human sacrifices, the curator explains Aztec practices while gesturing and referring to the displays around them. These are however not Aztec, but North-Western Native American costumes and artifacts made at least 400 years later by a different culture thousands of miles to the North.
The closeup of the newspaper headline shows the word "existence" misspelled as "existance."
The sun bather screams as the beast swoops in on her even though her eyes are covered and there's no sound coming from it.
When the police open fire on the monster, the sound effects for their machine guns include science-fiction blaster effects in amongst the sounds of gun fire. Ricochets can also be heard when there is nothing for the bullets to be bouncing off from to make such a noise.
In the opening scene inside the business woman's office in the Chrysler Building, as she passes by the window, a man in a blue shirt with his arms folded, can be seen to the extreme right of the frame.