I found this quite interesting, and it could be something. On the rooftop, Sherlock says something like "it's all a magic trick." And you know how, before John gets the call that Mrs. Hudson's been "shot," he's playing with a stress ball. My younger sister said that she read somewhere about a magic trick where if you rub a stress ball on your arm a certain way, it makes it seem like you don't have a pulse (is this true? I'm taking her word on it). So when John then takes his pulse after he's fallen off the building, he can't read it.
...are we onto something here? Do you think?
(sorry if you guys have thought of this before!)