There are closed tests where Robot does try to click somewhere too close to the frame, in two pixels maybe, to open menu, pass a mouse event etc. This area is reserved on Mac -- I would say even commandeered since developer is not restricted to put there anything but cannot use it -- for resize. On Mac OS before 10.7 only bottom right corner was used for resize. On Lion, whole of the border area is unavailable for Robot.
If the first test will be fixed fast, we'll create new bugs and see-also them here.