Obviously, Jaynes was talking about a reasonable prior probability you had before you flipped the coin. And he was talking about flipping the coin in a way tha

Obviously, Jaynes was talking about a reasonable prior probability you had before you flipped the coin. And he was talking about flipping the coin in a way that the head/tail outcome is completely unpredictable for you (50/50) probability, minus epsilon for weird stuff like landing on the edge. He later addresses the coin flip problem more rigorously. Besides, once you have observed the outcome, it's probability is not 1. You could have misremembered the sequence, or you could have missed a flip, or otherwise done your observation wrong. The probability of making at least one such error with 1000 coin flips in an informal setting is actually quite high.