As for me, I remember way back earlier in Reach forums when people were discussing the time travel theory: A theory that linked the Halo 3 legendary ending to him possibly traveling back in to planet reach during the fall.
[Edited on 10.05.2012 8:25 PM PDT]
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Oh, I remember someone said that Master Chief was travelling back in time to CE at some point. This was because of the [url=http://www.youtube.com/watch?v=m7wU1fpDYvQ]Two Betrayals Cutscene[/url]where 343GS says to the Chief, "Why would you hesitate to do what you have already done?" and "Last time, you asked me, if it were my choice, would I do it? Having had considerable time to ponder your query, my answer has not changed." So the guy was saying that the MC had travelled back in time to that mission again. Pretty crazy...