diberry - 5:26 pm on Apr 12, 2013 (gmt 0) [edited by: diberry at 5:49 pm (utc) on Apr 12, 2013]
Gotcha. Okay, here's the deal.
If Penguin is all about backlinks, then it follows that it's something to do with my backlinks that I never intended as spam, but it's looking like spam to Penguin, right? So I look at the page most harmed to see what's unique about it from (a) all my other pages and (b) all my other sites which have never been penalized or received unnatural link notices either. All I can see is that I had close to 100 trackbacks on this post, all obviously with identical anchor text. That sure sounds spammy (unless you're aware that Wordpress would do this by default if you posted a page with over 100 links on it). Are you assuming Penguin included a "free pass" filter for trackbacks to Wordpress sites? Because I'm not. I think this could totally explain why, despite my totally cowardly white-hatted avoidance of any intentional spamming practices, Penguin pegged me as a spammer. And in a year, neither I nor anyone else has managed to come up with a better explanation than this one.
Robert_Charlton had one of the most compelling suggestions: that perhaps I wasn't actually penalized directly by Penguin, but that my BACKLINKS had been devalued because those people had been penalized by it. Well, it sure didn't look like many of my backlinkers had been hurt by Penguin, so that seemed to rule out his suggestion. But if Penguin included the concept that a really huge number of no-follow/trackback links to a single page = spam, which doesn't seem unreasonable to me, then this page would have been spam under its definition.
[edited by: diberry at 5:49 pm (utc) on Apr 12, 2013]