The professor is in Kelly's MIND, reliving a memory with him...not only do the strict rules of reality not apply, the director made a POINT of showing Kelly reappearing in the vacated chair to illustrate Xavier's perspective. This is in no way a slipup.
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Aquila writes:
I agree, the fact that Senator Kelley had vanished from the chair mere illustrated that it was only Xavier's point of view of the situation and NOT reality.