I also have this problem on 10.10 maverick meerkat.
I was ripping an audio cd with rubyripper and wanted to see if rubyripper actually used multiple threads (I'd set it to 3, and I have a quadcore cpu) but then I couldn't close the System Monitor window anymore.
When rubyripper was finished, I still couldn't close the window, BUT
after opening the cd tray the window vanished without me touching it... :S
(Mind you, this is not a duplicate of bug #692420, since selecting Quit from menu item Monitor also doesn't close the window)