OMG, when Frank Oz's hand actually tears through the felt of the head, and his fingers are there just flapping away as the "mouth". And keeps going for a few moments before they realized...
It's like a muppet version of "Hellraiser" for just a moment, ROFL!
The one where they finish the whole take perfectly, then Fozzie's eye just falls off... "
plop". My co-workers came over to check that one out. They were in tears.