When Tomas enters the insurance company offices to meet Federico, the letters on the glass doors are placed so they can be read from the inside of the building. Normally, they should be placed the other way.
The first time wee see the Russian roulette 5-bullet variation, in Ucanca, when Samuel picks up the revolver then slides out the cylinder to spin it, he does it in a way that would have caused all bullets to fall off, yet none does. That would have been impossible.
While taking Polaroid's of the captives, a bright flash is used to photograph through a glass window. When we later see the Polaroid's there is no flare, which would have been all that was captured on the film.