Another reason to hope Microsoft gets it right in IE 7, seeing as they are reluctant to fix IE 6: I was making changes to a website for a client and mostly tested the site in Mozilla Firebird 0.6 - I forgot to test the site in IE, but that was a fault on my part on thinking it would be alright. It wasn't. The JS popup loads an invalid page because of the query string, which had become file.php?event_id=3®_id=12345.
Apparently, IE was being _too_ smart in converting the ® into ® even though the ending semi-colon ';' was missing. Changing to &reg_id didn't help either. It is a subtle bug though, since it only happens when loading via window.open(this.href). I had to fix it by making reg_id the first query string parameter.