Revealing mistake: When everyone jumps from the plane, all of the stunt doubles are easily seen - [Dr. Elliot's stunt double is ridiculously easy to spot!] (00:37:00)
Revealing mistake: In the shot of a cargo box falling from the plane, a shadow from the plane is cast on the trees below, but the cargo box doesn't cast one. (00:38:20)
Revealing mistake: Richard takes a photo of a Silverback Gorilla in the forest, however the shutter is clearly closed as he supposedly takes the photo. (01:03:25)
Revealing mistake: When the group is in the mines and there is a cave in, one of the porters is hit on the back by a (fake) rock, which bounces off. (01:21:05)
Revealing mistake: When the missile destroys the second airplane after everyone has escaped, the airplane suddenly stops moving forward. In the next shot where we see a close-up of the explosion, it is also stationary against the sky.
Answer: Seconds after he drops dead, an ape comes from where he'd been running from and throws a human brain at the living humans. This is Richard's brain, torn from his head. In the context of the film and employing a (fair) bit of suspension of disbelief, he must have had enough brain stem (where reflexes are governed) left to run and scream a bit before dropping dead. In reality, of course, this is an extremely unlikely if not impossible scenario, invented by the screenwriter for the purposes of a gruesome death. Perhaps inspired by stories like Mike the Headless Chicken, an animal which survived for months after having most of its head/brain chopped off.
That wasn't a brain that the gorilla threw; it was the head of one of the two porters with whom Richard remained outside the cave.
Phaneron ★