I've had something similar happen on mine as well in Yosemite, sometimes if I have ME running and unplug the HDMI feed while full screen is still active the video card freaks out. I have to to plug monitor back in and the extended desktop finally comes back up w/ the full screen output. Then I stop ME, and unplug the HDMI and all returns to normal. I'm still running 2.7.1, never had it happen in earlier OS X so I figure it's a 10.10.x quirk.