LSD, like any other strong hallucinogen, is not a drug to be taken in close proximity to large groups of people. The large amount of chatter and noise would create intense paranoia and fear in the user, and the person could very probably have a nervous breakdown.
This phrase is a play on words, originating from the movie Apocalypse Now. One of the main characters, a drug user, claims that he had been saving a tab of acid for a rainy day. Before entering a warzone, he claims that he had dropped the tab, and proceeds to act like somone who is tripping balls.
Hippie 2: Jesus! That must have been apocalypse acid!