See what scrapeshield does by checking to see if you are using a real browser, essentially checking your browser for specific errors. Suppose Chrome cannot handle an IFrame if there is a 303 error in the line, a certain web browser responds differently to different tests, so the webdriver should not react to this, forcing the system to say "We got an attacker, change the page!". I could be right, but not 100% sure, though ...
Additional source information:
I found most of this information in Defcon's talk about web sniffers and did not allow them to get the correct vulnerability information on the server, he also made a web browser identifier in PHP.
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