If one accidentally presses F12 while comparing two files the ensuing Post-It-mode becomes stuck. One cannot toggle back by pressing F12 again, one cannot change over to another open program via task-bar or Alt+Tab. One can open new programs via the startmenu but cannot get to them either.
The only way to get out of this is to close Notepad++ (either through right-clicking on the taskbar or through Alt+F4).
This problem occures on Windows 7 64bit. I've got no means to check other OS.