I'm proud to say that I've never been responsible for a site that has been punished by any of the Google Panda/Penguin updates.
I've seen my fair share of Google fallout though, usually brought forth by someone after they've seen their traffic tank. They don't know what happened, why, or much less how to fix it.
Truth be told, I'm surprised by how often I've been presented with this scenario. Every single time it's boiled down to using bad SEO or outdated SEO tactics that they consider tried and true but are no longer valid.
As Mark Jackson stated ...