Earlier today, we noticed a bug report for a very strange issue with multiple user accounts on the Nexus 7. Apparently, certain applications are displaying the underlying screen - be it the wallpaper, app tray, or the Play Store (depending on where the app was launched from) - through the foreground app. But here's the kicker: it's only happening on secondary accounts. Yeah, it's confusing and strange.
There's also a Reddit thread that confirms this is happening, and it seems to only be affecting apps that call on some sort of transparency.