iconv() Detected an illegal character in input string

[eluser]WanWizard[/eluser]
For every page request, $_GET, $_POST and $_COOKIE is cleaned. Somewhere in there is something that is not ASCII (and thus triggers the UTF8 functions), but not UTF-8 as well, which causes iconv to choke.

Weird thing though is that altough '//IGNORE' is used, it still generates the notice error...