Don't know if you can call this a bug really :
When using [quote] the name is automaticly enclosed in [noparse] tags, so if someone quotes someone who has "]" in his/her username, the quote parser, won't get confused and think that the quote ends there.
That's why the url tag wont work there, because it's not parsed.
Edit: By looking again into it, it seems that the name is not enclosed in noparse tags
/ 
