The film's themes and meaning has oft been debated with many interpretations being put forward such that it's the Orpheus and Eurydice myth; that it's the Doctor Jekyll and Mr Hyde yarn; that it's really the story Mary and Jesus. The film's writer Paddy Chayefsky once revealed in an interview that "Altered States" was actually really a love story.
During the hallucination sequence in the cave, the pyrotechnic charges underneath the mushroom-shaped rock are visible as it elevates, as is the wire lifting it up.
When the Brujo tells Eccheverria that he'll allow Eddie to participate in the ceremony he walks off. Although in only a matter of seconds he's far enough away that they have to run quite a distance to catch up to him to ask him some further questions, this is consistent with other literary and screen depictions of shamans having "spooky" abilities, sure-footedness and being surprisingly limber for their age. Rather than an error in continuity, this seems to be a dramatic device.
At one point, the character of Mason Parrish refers to the "nuc-you-lus" of a cell (as opposed to the correct "nuc-lee-us".) While it's a commonly mispronounced word, it's very unlikely that a scientist with Parrish's credentials would make that mistake on a word that they likely read, hear and use practically every day.