You know, I'm rather annoyed at Opera.
One of the things I learned from ciwah is that MSIE is not a browser because it ignores published standards, one of them being "the Content-type header which a web server provides is authoritative" (MSIE ignores it in some cases, including text/plain and—I believe—application/octet-stream and attempts to "sniff" the content type by inspecting the data).
I've seen Opera do this too. For example, when viewing some Zilla attachments containing bits of HTML code embedded in the patch, it'll display the page as HTML even though the web server says it's "text/plain".
I'm even more disappointed since I thought Opera marketted itself as being one of the more standards-compliant browsers around. (And besides, rendering text with some embedded HTML tags as HTML screws up the text so you can't read it.)