Dragging a window to a space doesn't work the first time In Mountain Lion (at least, maybe elsewhere too):


Enter Mission Control to see all windows, and all spaces.
Drag a window or a stack of windows to a space


For me, the first attempt at this fails reliably; the space doesn't get darkened focus appearance, window springs back to original location on mouseup: 

The second attempt succeeds reliably; space darkens, window moves on mouseup: 

Happens with all applications, on a very fresh install of Mountain Lion, with no UI hack utilities installed, etc. Very annoying at start of workday when trying to place apps in spaces. Is this a bug or some sort of feature I am not aware of?

Update: After upgrade to 10.8.2, this problem is resolved.
Confirmed: the mountain lion 10.8.2 update resolves this problem.