I don't usually run Google searches on FireFox only to see some detailed information as provided by SEOQuake. Yet, today I was searching for some potential answer to a problem I'm running into with MS-Access which crashes a database and ran those on FireFox instead of my favorite: SeaMonkey.
As I was testing, all of a sudden I got the screenshot as shown in the figure below. Although it did not feel like I was sending automatic requests to Google, the SEOQuake toolbar does so on all the results! That means each time I do a search "I" send at least 11 requests to Google. After 3 or 4 searches with pretty much the same terms each time, that's well over 40 requests within 2 or 3 seconds.
That's the first time I got this screen, I'll have to think of turning off the SEOQuake bar whenever I do intensive searches like these.
By the way, our Snap! Websites system has a similar protection: MO Anti-pounding. Most of our websites, if hit too many times, will give you an Error 500 asking you to slow down. This ensures everyone can enjoy the experience of using our systems.