Having it fail doesn't work anymore, not with PSR-4. There should not be a situation where someone wrote a PSR-0 autoloader that prevents the PSR-4 autoloader from having a chance to load stuff.

Additionally, there is nothing in the standard that says the autoloader can or should throw an error. Without the standard giving permission to throw an error, and considering the standard says it exists for "painless autoloader interoperability", I would say this example autoloader actually violates the standard (in it's goals if not it's words) unless we accept this pull request (after the formatting has been fixed, of course).