I woke up this morning wanting to write a blog post, then I found that someone else had already written it. Thank you, Andrew.
If you succeed in getting a certificate misissued to you, then that has the opportunity to be a great learning experience for the site, the CA, the CAB Forum, or all three. Testing security is, to my mind, generally a good thing. But publishing the private key turns it from a great learning experience into a browser emergency update situation (at least at the moment, in Firefox, although we are working to make this easier with OneCRL).
Friends don’t publish private keys for certs for friends’ domain names. Don’t be that guy. :-)