Factual error: During the scene where everyone runs outside, the Statue of Liberty head crashes into the street. The head is scaled too small and its size can be compared to the people taking pictures. In real life the statue head is large enough for people to walk into (17' from chin to cranium).
Audio problem: When the Statue of Liberty's head strikes the top of a distant building before landing in the street, sight and sound are simultaneous. There should have been an audible lag in the sound due to the speed of sound being only about 1000 feet per second. The absence of this lag would be acceptable in most movies but is inconsistent with the amateur home movie theme of Cloverfield.
Continuity mistake: In the scene where Hud, Jason, and Rob are hiding in the grocery store, you can see several items toppled over on the shelves above Jason's head. When the lights start to flicker, you notice almost all the objects are now standing upright, only to be toppled over again. (00:21:45)
Continuity mistake: In the scene where Rob is walking on the street in the direction of Beth's apartment (just after leaving the electronics store), you see on the right-hand side of the street the orange framework of a store front. There are parked cars there, including a black Chevy Malibu followed by a white Mercedes, which Rob stands in front of as he points towards the military on the next block. Two scenes later they are (briefly) walking the opposite direction on the same street, which would be away from Beth's apartment. The orange framework is now on the left side and the direction of the parked vehicles is reversed. The black Malibu and white Mercedes are in different positions (as are the other cars on the street). I don't know New York City, so I don't know if the buildings in the background are different. Also, a man in a yellow shirt and tan pants runs past them in both scenes. It is the same man both times.
Factual error: In the first scene where the Army attacks the monster, the first tank that rolls by is actually a British FV-433 Abbot self-propelled gun made during the Vietnam war years, not an M109 Paladin which most people believe. (Paladin has a longer gun barrel, and has rear turret overhang.) And a few seconds later we see, next to the M1 Abrams, what appears to be an MLRS system firing, but is actually some kind of truck with a multiple rocket launcher, much like the Russian BM21, not the U.S. tracked MLRS system or the M1097 Avenger (which I originally thought it was).
Revealing mistake: As Rob, Hud, and Lily are rescuing Beth from her apartment, the view outside the window is tilted roughly 45°. This is due to Beth's apartment building being partially collapsed against another building, demonstrated previously by an exterior shot. However, the characters are moving within the apartment with no difficulty or apparent change in gravity, even though they should be struggling to remain upright in a significantly tilted apartment. This makes it clear that the scene outside has been added using chroma key technology, and the camera has simply been tilted to create the illusion that they are on an angle. (01:01:30 - 01:01:52)
Other mistake: When the initial attack happens, it shows a gradual blackout of the city. Yet in all the following scenes, the lights are on.
Chosen answer: The actors and crew were kept in the dark as much as possible, unless the information was needed for their performance. They didn't know what the parasites looked like until they began filming the subway scene. Actual-size, realistic puppets had been made for filming but were decided to look too artificial and slow on camera, but it gave the actors a chance to see what they would be running from, how their jaws and legs worked, etc. As for the main monster ("Clover"), actors would only know what was in the script and what the director told them.