NSURLErrorCancelled is returned "when an asynchronous load is canceled", which in the real world happens a lot with things like Facebook like iframes. Everything I've read suggests it can be safely ignored when in `didFailProvisionalLoadWithError`.
As noted in #13, the method we used to capture the contents of the window didn't work with webkit-transform. Nate Weaver (@wevar) provided some nifty code that should fix this when run on Mac OS X 10.5 or later whilst still letting the script run on older versions. Only problem is that I've only been able to test this on 10.7, 10.6 and 10.5. I don't think many people are running webkit2png on older OS versions so this should be OK.