SPGetCurrentUser does a "screen scrape" of the /_layouts/userdisp.aspx page, as described in the docs. If a user hasn't "touched" the Site Collection, they may not be in the User Information List which feeds that page. However, since the current user is logged into the Site Collection, I'm not sure how that could be. It's possible that it's simply a timing issue, but I've never had any reports of it that I can recall.
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