One thing I've thought about is whether observations in the present can influence past events. I'm thinking it must be so, though probably only on a microscopic level.
Choice of how to measure + physical system -> Observations -> Interpretation of observations -> History
The choice of what and how to measure will influence the history you conclude, but that is true of actual "Caesar and Napoleon" history too, and in that case it's definitely not that past events are being changed, instead it is your knowledge of them. A really interesting principle is that any philosophical question that can be phrased without referring to ideas that only exist in quantum mechanics can usually be answered without referring to them.
