That is the favicon for the Google Apps Admin Console - <a href="https://admin.google.com/" rel="nofollow">https://admin.google.com/</a> . I suspect the bug only happens for Google Apps accounts (it happens on my Google Apps, not on my personal Gmail).
There's also a Gmail Labs feature to display unread message count in the favicon ("Unread message icon by Manu C"). That is both super-useful and fixes the favicon issue.
I noticed this about a week back in our business account. If you view the page source, the favicon actually points to a favicon in a your company's folder (for Google Apps). I have a feeling they will allow us to customize the icon shortly from their admin panel.
Oh the ugly spanner icon turned up on my works gmail a week or two back and its been bugging me to bits!<p>Why was it showing a spanner? It made no sense to me. A bug in gmail perhaps? But if it affects <i>so</i> many people, why haven't google noticed it before then?
I stopped using the "Unread Message Icon" a while back because it would often get "stuck". How does a screw-up like this go unfixed for so long anyway? It's been at least a week. It's only a favicon!
Or you can just use this awesome userscript (screenhost are dead btw ..) <a href="http://userscripts.org/scripts/show/24430" rel="nofollow">http://userscripts.org/scripts/show/24430</a>