Mission Control bug on Mountain Lion: Dragging Windows to Desktops Fails Regularly

When I activate Mission Control and try to drag drag, say, two windows to another desktop, the first drag works fine. I can drag the window to the other desktop's thumbnail, which darkens, and then release the mouse, and the window moves to that desktop just as it should.

When I try to drag the second window, however, the recipient space won't darken—it doesn't register that I'm dragging the window over it—and when I release the mouse button, the window will snap back to its original position as though I'd dragged it to some other part of the screen.

When I initiate another drag on the second window (the third drag total) it'll work properly. Then, I'll drag another window and it won't work. It occurs every other time I drag a window, with perfect regularity, as far as I can tell.

Other points:

• Non-working drags doesn't have to be over a space for the next drag to work. I could drag a window an inch, release it, and then pick it up and drag it to another desktop just fine.
• The "new desktop" button never seems to suffer from this problem.
• The problem also occurs on the guest account.

Any ideas how to fix it, or why it's happening? Any help is appreciated.

This has been killing me, too. But I went into the MC preferences pane and unchecked "Group windows by application." It worked.