My probability of ending up the original couldn't have been 0.5^99, that's effectively impossible, less than the probability of hallucinating this whole conversation.

Does anyone have a sense of what the lower limit is on meaningful probability estimates for individual anticipation? Right, like there should be some probability p(E) where, upon experiencing E, even a relatively sane and well-balanced person ought to predict that the actual state of the world is ~E, because p(I'm Crazy or I've Misunderstood) >> p(E).

More to the point, p(E) should be roughly constant across apparently sane people; I would guess that the probability of hallucination doesn't vary by much more than a factor of 10 among people who have no reason to expect that they are hallucinating. 10^1 might be small relative to whatever the Minimum Probability turns out to be.