I'm not sure this could be considered a "bug"; rather a side effect of the way Visual Studio 2012 draws the glowing window borders. Each of the top, left, right, and bottom border are actually separate windows, with the class name "VisualStudioGlowWindow"; you can use Spy++ to verify this.
According to Spy++, the four drop shadow windows are hidden when the main window is maximized. Because Aero-docking the main window to one of the edges of the screen isn't a fully "maximized" window state, the code that manages the shadow windows doesn't hide them and they end up "leaking" into the adjacent monitor.
It would be interesting to see if this issue remains on Windows 10, as the glowing border feature is built-in to the operating system in that version and should manage itself accordingly when a window is maximized. Unfortunately, I do not have a multi-monitor setup on Windows 10 to test this.