Hi Frederick,
I’ve recently come across this issue on a 3rd party site as well and was wondering if it may be related to the very mild browser-sniffing you’re doing in lib\W3\PgCache.php and lib\Minify\HTTP\Encoder.php where you seem to lump Opera in with the old IEs? Not had time to test this on a site I control directly, but may be worth a look.
(full disclosure: I work for Opera as Web Evangelist)
