Mountain Lion not switching displays

So for a year or so I've run my MacBook Pro on it's built in monitor at work, and then plugged it into an external monitor when I get home. No issues. I connect via the thunderbolt port. After upgrading to Mountain Lion, no problem on the external monitor at home, but when I disconnect my laptop from the thunderbolt port (and all my other cabling) and take it to work, when I open it up at work, it brings up the display correctly for a second or two, and then the screen switches to pure weirdness, I can't even describe it well. Just unreadable. Like the full screen goes into a test screen mode or something. can't make out icons, text, nothing, just a complete mess. At that point all I can do is hold the power switch down until the unit powers off, then reboot.

What's going on here?