Inconsistent urldecoding

Path decoding appears not to behave identically when performed with or without clean URLs. For URLs supposed to be equivalent, hook_menu implementations may not receive identical parameters.

This happens for URLs with double-encoded values, as may be needed for specific values like the ampersand or percent. For instance, consider passing an ampersand as a value in a URL.

Using clean URLs: http://www.example.com/path/%2526
Using standard URLs: http://www.example.com/?q=path/%2526

In the first case, the hook_menu implementation receives the parameter as an actual ampersand, already decoded, but in the second case, it receives it as the non-urldecoded string "%2526".

This inconsistency exists in 5.0 RC1, 4.7.4, 4.6.10, and quite possibly earlier versions too.