According to a 2009 blog post[1]:

> Thanks to our new servers, known bad keys[2] from Debian and Ubuntu are now blacklisted. We estimate that about 1000 keys in our database were impacted by this. If you get authentication errors using keys that worked a day ago, please double-check that they are not on our blacklist. If they are, you should ensure your software is updated and generate new keys. We’ve got a guide to help you out with this.

[1] https://github.com/blog/500-state-of-the-hub-rackspace-day-0

[2] https://github.com/blog/63-ssh-keys-generated-on-debian-ubun...