Apparently, when you “Stop all crawls” in the Shared Services Administration: Default Web Site > Search Settings > Content Sources of SharePoint Central Administration, the next crawl will be a Full Crawl. The warning told me! Wait, I mean the warning that should have been there.
From TechNet:
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On the Manage Content Sources page, point to the content source you want to crawl, click the arrow that appears, and then click Stop Crawl on the menu that appears.
Note: A message appears warning you that stopping this crawl will result in the next crawl of this content source being a full crawl.
So that's why you see a full crawl when it should've been incremental. Or maybe I'm the only one who doesn't get this error?