I'm by no means an expert on Fringe but I understand that the 19th episode of each series has been given over to an "experimental" or "playful" chapter, and that last year we got Lysergic Acid Diethylamide. In that story, the use of LSD led to an animated sequence.
And so it was again last night in the episode, Black Blotter. Not the 19th this time, but the 9th - we're into the final season of just 13 parts.
They're certainly having a lot of fun before the curtain comes down, though. Here's a video of what happened when Walter dropped acid.
Quite a fun pastiche of Terry Gilliam's Monty Python animations but not, I have been told, anything at all like any kind of acid trip.
Apparently the Gilliam work that best represents the effects of LSD are some of the sequences in Fear and Loathing in Las Vegas - rather unsurprisingly. Gilliam has noted, mind, that he's never taken acid and they were all worked up from imagination, not from memory.