Mistake- dead man breathing!
Peter is killed whilst carrying the President and firing at the Simba in the gulch.
But as the men peel away, (i rewound this several times) the dead Peter (Kruger) stomach rises slowly.
Not so much a mistake as a goof, I don't think the actor thought afterwards "does a dead man breathe or not breathe?" as would imply a mistake.
Goofs like that usually happen when a scene has to be shot too many times and a dead person or a person eating a bit from a donut for the 12th time does a half-arsed attempt. Good directors get the best from their actors in 1 or 2 takes, crap directors force shoot after shoot.
Opinions are just onions with pi in them.