It happens sometimes that the yellow circle in the top left of the windows, for minimizing, becomes gray for some windows. In particular it is gray for Finder, Safari, Calendar, iTerm2, Xchat Azure, Chrome, VirtualBox, and is still yellow for other windows, for example Mail, KeePassX, NoteBook, FreeMind, DjView, Lyx.
Obviously, when it is gray it is disabled.
The situation comes back to normal apparently without any reason.
The OS is Mountain Lion 10.8.5, and this is a screenshot