First spotted by Britoid on The Verge Forums, the image shows a tablet that looks to have a screen that's about 8 inches. It's running KitKat (you can tell from the status bar icons), and it has on-screen navigation buttons. Unlike the current Nexus tablet, the Nexus 7, the bezels look to be very compact. The Nexus 10 is a year old now, and many expected Google to have announced a replacement for it by now, but a smaller 8-inch tablet would be a surprise.
Notably, this isn't the first time Google has 'accidentally' leaked its upcoming devices. A promotional video ad for KitKat revealed the Nexus 5 smartphone. After it was noticed, Google took down the video.
Take a look at the photo below...