[MrWookie]

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Hiro can't change the past,

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Except he did change the past.

There's a photo of Hiro with the waitress, taken six months ago, that would not exist without Hiro's time travel powers.

That's not changing the past in a big way, but it is changing the past.

In addition, if Hiro can't change the past, why did future-Hiro travel into the past to talk to Peter on the subway?

Hiro's wrong, he's just frustrated by a coincidence - That the waitress also had a brain annurism as well as his lack of mastery over his powers (the accidental time jump).

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We're not even sure, though, that the memorial in the restaurant is for the waitress as murdered by Sylar, not the waitress as victim of an aneurysm. I'm not sure, but I think there's a possibility that Hiro did in fact save her from Sylar, but not from her clot.