There's also the possibility that it does happen often and no one notices, of course.

Makes me wonder how you'd approach the "how do I find bad certs" problem.

You'd think that an entity which, say, scrapes a large portion of the Web on a regular basis ... might be able to detect such things.

Meanwhile, there's CertWatch http://certwatch.simos.info/ and The Convergence Project http://convergence.io/

It was easily noticeable thanks to Chrome not trusting it in the first place (which also would have prevented spoofing the download server, though I suspect there's more to protect that than just a key).