I believe this was introduced by bug 553483. I'm not sure if there's any visible effects, but if it is an expected error it should be handled or otherwise fixed.
Think this is the result of the changes in bug 553483 and bug 562760 not taking into account one another. There's a patch in bug 562760 which should solve this, among other things.
The follow-up patch for Bug 562760 landed as http://hg.mozilla.org/mozilla-central/rev/63f5ac31b8c4 fixed this bug.
Verified fixed with Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.3a6pre) Gecko/20100611 Minefield/3.7a6pre We don't wanna have a Litmus test and I would think the same for an automated test. Lets set in-testsuite? to be verified by devs.
Agreed - given this was just a collision of patches, I don't think its worth trying to test this.