I have a custom route loader. When an exception is thrown from within my loader, in this case it's a ContextErrorException for an invalid array key, the browser always shows a CircularReferenceException.

Digging into the Symfony code, I can see the problem.

On line 104 of the Component\Config\Loader\FileLoader class, the resource is added as a key to a static array. When an exception is thrown within the custom loader (line 106), that key is never cleared. Then, the WebDebugToolbar tries generating it's own routes. Because that static array still contains valid data, the first resource routing_dev.yml causes the circular reference to be thrown.

I think the key should also be unset within the catch block.