Well, this kind of thing can happen if the video buffer is being rapidly updated at the same time we're copying data from it. For example, if the visualizer is running, it's writing to the display so quickly that it's hard to capture one "update" before another one has been written to it by the GPU.
During normal operation, though, you shouldn't ever see this kind of thing. Have you tried rebooting the headless server?
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--Dave Nanian
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