I hate scrolling through the Q&A section and seeing a 0-response question surrounded by so many answered and responded-to ones. Whenever it's one of my questions with no responses I always get the feeling I'm being ignored. Know, CJ, that I'm not ignoring you, I just read your question and honestly have not a clue.
It sounds like it could be a campaign bug. It happens, I've had them happen to me a few times. I've had duplicate URLs tagged as duplicate content one crawl, changed nothing, then next crawl it was fine. Sometimes the crawler hiccups. If I were you I'd pull a manual crawl (research tools), or use Xenu to run one, and see if it's there too. If it is, you can see the referring URL, or at least more information. I could be of more assistance then.
If it doesn't appear in those, well, campaign crawler hiccup/
