It seems there is a bug in moving Progress Form code where if a control is visible only slightly Progress form will not move out of the way for mouse to click this control. I nocied it once or twice. I'll try to recreate the problem.
EDIT:
I can't seem to recreate it on my machine using Calculator, so i guess that it's something wrong with my code or some timing issue on a virtual machine. I've disabled Progress Form so I shouldn't run into it again.