http://www.nydailynews.com/new-york/obligations-affidavit-su... - This details at least one court case of USCIS enforcing an Affidavit of Support, and was literally the first Google result on which I clicked. They do mention that it's rare, but it happens.
No, that's an immigrant enforcing the affidavit of support as a third party beneficiary against the sponsor, I've heard of cases like that, though they are relatively rare.
What I was talking about was where a government agency sues to recoup funds when a beneficiary uses in a means tested program. That I've never seen and google doesn't seem to come up with any examples.
