F.R.E.S.H Edward's Weblog

In general, a character is reserved if the semantics of the URI changes if the character is replaced with its escaped US-ASCII encoding. -- section 2.2

Implementers should be careful not to escape or unescape the same string more than once, since unescaping an already unescaped string might lead to misinterpreting a percent data character as another escaped character, or vice versa in the case of escaping an already escaped string. -- section 2.4.2