Well, actually, it means that if you drag fast over a small picturebox, it would trigger the FORM's MouseMove Event because the mouse is already outside.

The clearer way is that when you have a 3-pixel picbox, it's not natural for a user to take 40 secs to move 120 pixels, so that may be a problem. You should add the same thing in PICTUREBOX2_Mousemove and (Add to Form_Mousemove a code that moves the picbox2 to a edge of picbox1).