The stats code introduced yesterday had a bug which only triggered about 24 hours after it had launched. It kicked in for different blogs at different times, but the result was that starting sometime last night you probably started to see really obvious spam getting past your Akismet filters.
All of our systems were up and running at the time, and spam was being correctly identified, which is why our monitoring systems didn’t notify us of the problem. However, there was extra junk being included with the response which broke the API call, which expects only “true” or “false”.
I’m really sorry about this, when things are working smoothly it’s easy to forget how much vile junk is actually being blocked day to day. I’m going to go through my blogs now and double‑check that no spam actually got published, and I’d suggest other folks do the same. The “mass edit” mode under “Comments” should be useful for WordPress users.
I’ve also taken steps to make sure on the Akismet side this kind of error can’t happen again.