One of my sites had a page that was sitting at #1 for a term and #6 for the more competitive version of the search term. Over the past week it has just dropped out of the index for these terms - the page is still indexed and still receives traffic for related terms so it sounds to me like a term-specific penalty.
I actually know exactly why this has happened - it's because of an unnatural anchor text profile that is the result of some lazy link building last year (to be honest i'm surprised I got away with it for this long!).
I'm going to assume it's algorithmic for now (rather than manual) and I'm trying to figure out the best way to fix it. Is the most efficient (and fastest) way to drip feed new (clean) links to the page? Or try and have old links changed? Or have the old links dropped?
I'm keen to hear from anyone with experience of recovering from this kind of penalty!