This is one of the only times it has happened to me, but in this case I would say the model had an actual phobia of being tickled and I stopped the shoot early.
www.ticklingforum.com/showthread.ph...-Unbearably-Ticklish-Cat!!-(M-FF-FEET-NYLONS)
She was distressed to the point of tears and trembling even at the idea of being tickled and it was a fairly uncomfortable situation - I'll never know why she agreed to do the shoot in the first place!
Just a comment..... she looks so much like one of my nieces it's scary! Not her, though. Her hair is fire red! Plus, my niece is way skinnier.... almost a toothpick...