$ wget 'https://bugs.gentoo.org/attachment.cgi?id=287485&action=diff&context=patch&collapsed=&headers=1&format=raw' and you'll notice that the created .patch file doesn't contain any added lines, just removals. You'll also notice the line counts don't match. That's because bugzie, for some reason, removed all lines starting with '++'. 'Accept' or 'User-Agent' may be relevant, as Opera gets correct patch contents.
(In reply to comment #0) > 'Accept' or 'User-Agent' may be relevant, as Opera gets correct patch contents. Hrm, my bad. I must've been using another URL. With this one, it doesn't.
The URL you're quoting is the "Raw Unified Diff" output of Bugzilla. You may not assume this returns the same contents as the raw attachment. The issue is known. At any rate, you should be using the standard attachment URL (reachable from the attachment display page via the "View" link). *** This bug has been marked as a duplicate of bug 251286 ***