Yes, that's exactly what happened! I just watched a youtube clip that shows it quite well (Phish - 04.03.98 - Loving Cup -- Antelope (intro)). Around the 6:20 mark, a shirtless guy with a tiger backpack springs onto the stage out of nowhere. He skips along and pumps his fists in the air until Carini confronts him, forcing him to jump back into the audience which was feet from where I was standing. Funny shit, great show!