These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Special Effects Failure: The hallucinatory sequences (of which there are many) are brilliant, but William Hurt in a fur suit just doesn't scream Australopithecus, especially considering that Stanley Kubrick made more convincing Ape-like human ancestors with late-sixties special effects in 2001. Kubrick probably had more money to work with though: Ken Russel's "primitive hominid" character looks like something out of a 1930's serial.
Actually, some of the hallucinations look like what they are: drops of paint exploding on Petri dishes. Bran Ferren replaced John Dykstra after the latter quit the film, one of the many things that went wrong.