In trying to diagnose a userscript bug, I realised that some code was using the old, domain-based API 1.1. Figuring that Patents rightfully had no reason to support the deprecated API, I typed in the URL to verify and was surprised to see this:

...instead of a 404 page or the API 1.1 documentation. This seems to only happen for the API subdomain, and requires you to input the broken URL manually, so it's admittedly a very minor issue. All the same, it should probably return something more appropriate.