When Vasquez checks his pistol at the end, there are only two cartridges in the cylinder; the remaining four chambers are empty. A revolver doesn't eject spent cartridges, so the other chambers should still contain spent shells.
Adding to the comments about the revolver: It's possible that a couple of the expended shells had been previously removed from the cylinder. However, Vasquez shoots Ellis just before the plane takes off, but there's no indication that he removed any spent shells and little time for him to have done so.
While they were flying straight and level, a few shots were shown out of the pilots window. The instrument gauge located at the top center is an artificial horizon. It is stuck in a hard left turn while they were flying level. This is repeated again when they were not in a hard left turn. This instrument gauge was static for effect only.
The passengers and crew look remarkably clean and well-groomed for people who have been marooned in a jungle for more than 3 weeks.
Aboard the plane, Professor Spengler tells Crimp that he seems to be a "maniac depressive", when what he obviously means is "manic depressive".
Despite continuous references to flying to Panama City and spending time there one of the pilots asks a passenger if this was her first trip to South America. Later the plane's course references flying over the Andes and putting down near the Amazon. Panama City is in Central America and certainly not in South America.
Using Lucille Ball as a character was a big mistake! Being that she would one day wind up marrying Desi Arnez and staring in "I Love Lucy" it had to be obvious to everyone that she would be chosen to survive!.