If I drag the xvidcap application window down the screen, the capture area moves down with it.
If I keep dragging, eventually, the bottom of the capture area hits the bottom of the screen, and stays there.
Capturing with the area in that position results in some corruption in the captured video - see the attachment for an example.
Capturing single frames rather than multi frames stops the problem happening.
In the attached example, cap_geometry is 1001x637+457+413, and screen size is 1680x1050
I have been using avant-window-navigator, with auto-hide enabled, which makes the bottom edge of the screen sensitive. I don't know if that's relevant.