Today, a mysterious new nub popped up next to my facebook chat panels. It is called "Quick Experiments". When I click on it, a chat-sized view opens up: 
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Google tells me something about Facebook ad experiments, but I'm not sure if that is what this view is about. Especially since I haven't ever bought Facebook ads.
What are Facebook Quick Experiments?
When logged into Facebook, choose Account->Privacy Settings. Select "Friends Only" on the left and apply the settings. Alternatively, you can choose the "Customize settings" link and further refine your privacy.
Once you've made the appropriate changes, visit http://www.reclaimprivacy.org/ (Edit: Dead Link) to have it scan your settings and make sure you have everything set up correctly.
According to Facebook help it is indeed 851 × 315 (in pixels). The minimum size you can upload is 399 × 150 (but then it'll be stretched to fit).
Cover photos are 851 pixels wide and 315 pixels tall. If you upload an image that's smaller than these dimensions, it will be stretched to this larger size. The image you upload must be at least 399 pixels wide and 150 pixels tall.
To get the fastest load times for your Page, upload an sRGB JPG file that's 851 pixels wide, 315 pixels tall and less than 100 kilobytes. For images with your logo or text, you may get a higher quality result by using a PNG file.
Best Answer
It was a Facebook glitch, where they accidentally released an internal tool that's usually used by Facebook staff to try out small code projects with small sections of the facebook user base, to all Facebook users.
They figured out the glitch and removed the tool a few hours after it first appeared.