Opera is actually rather famous for its setting of different user agents. Especially since the Bork Incident : ) Microsoft actively tried to give Opera users a crappy broken page even though Opera was able to display the page just fine. Since then, Opera came out with a useragent name of IE a lot of the time. Wells Fargo, a major server manufacturer (who was trying to get Opera as a client!), and Google have all blocked Opera at one time or another. The solution is to change the user agent. Magically, everything then works. I have also changed the user agent on one of my Firefoxes in order to access a page that was trying to block all non-Safari browsers. So I just called mine Safari. Done.
More recently, when Opera started stating that it is Operax.x, the old crapalicious scripts that sniffed for browser versions puked on "Opera10". Why? They expected only single digits before the decimal. This means "Opera 10" == "Opera 1" which of course is "too old", and thus the user gets the message "your browser is too old for this page, please install Internet Explorer because it's teh Awesome" or whatever.
so now they say 9.8 lawlz.
Normally, instead of useragent sniffing, one would do feature-detection... but in this case, you really do need to know at least the OS (not sure what the browser type has to do here).