Created attachment 731697 [details] email message demonstrating issue The commit in bug 762594 does not fully fix the issue. I have attached an email message for which the issue still occurs even after that commit. I regressed nightly builds and found that the first one in which the bug occurs is 2011-09-23. I'm betting that the commit in bug 679476 is responsible for introducing this bug.
Bug 745919 fixed this.