The image for the spellchecker in the message compose window in the classic skin is broken. It looks like the button does not have the correct image region defined.
This looks bad and we should get it fixed for beta. It's a new feature for this release and we shouldn't go out with a broken button for a high-profile new feature. If we don't have time in beta then it needs to definitely block final.
bug 207243? That one's for Composer. Looks like the patch there fixes it for mail compose. *** This bug has been marked as a duplicate of 207243 ***