Corrected entry: When we first see the helicopters taking the squad in, the rotor discs distinctly show 4 blades. After the interior "red-light" shots, the final exterior views of the helicopters, show them with 2-bladed rotors.
quatermass
13th Nov 2004
Correction: This isn't a continuity mistake. The helicopters are bog standard UH-1N 'Huey' helicopters, with a single, twin bladed rotor. When filming a rapidly spinning object it is possible to photograph it twice or more when exposing film at 24 frames per second, giving this weird effect. If the rotor blades were spinning at 48 revs per second, it would appear as if the helicopter had a four bladed rotor.