I pointed this out elsewhere but it seemed to get tangled in with another topic so I'll put it here instead.
The CAPTCHA window is X pixels from the top of the window, on a resolution of 800X600 and a few toolbars installed (it's not my machine) the user cannot see the entire window, I can imagine that since smartphones and netbooks are becoming smaller and more popular a shorter window may render the CAPTCHA impassable. The javascript behind the CAPTCHAs put them on a different layer so scrolling isn't possible either.
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