That is a known problem when you use an Aero Theme in Microsoft Windows 7 (and probably Vista, too). In that case, the window size actually changes by 1 pixel when the video recording starts.
We're working on that topic but as a workaround, you can switch off Aero (e.g. by selecting another theme in Control Oanel -> All Control Panel Items -> Personalization) before you start the video recording.
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