Google prevents automated queries
Sorry.google.com is the page Google serves you (or an application) when you fail to answer it's human-verification question. Google has instituted this new policy - randomly checking to see if queries submitted to it (ie. searches) are done by human beings (as opposed to software applications). The purpose of this is two-fold. On the one hand, blocking the search query and redirecting the user to sorry.google.com alerts the user to potential spyware on their PC; on the other hand, it prevents users from utilizing software to query the Google search engine.
Google protects you from spyware
On the sorry.google.com page, you'll see Google recommending you run anti-virus and anti-spyware checkers. Why? If spyware is present on your computer, it may be accessing the Internet, and will often execute Google queries. If Google detects this, instead of giving the application what it's looking for, it will block the request and serve the sorry.google.com page instead. On this page you'll see recommendation for you to scan and clean your PC in the event of an infection.
Google blocks Google ranking and utility tools
Just because you see the page sorry.google.com doesn't mean your PC is infected with spyware. It's also possible that you have a browser plugin (Firefox add-on) or software utility installed, such as a Google ranking checker, that performs regular Google queries. Since Google randomly executes human verification, the utility will get caught, and eventually the sorry.google.com page will get served.
While this reduces the load on Google's servers, it also prevents software and utilites that rely on the Google API and querying capabilities to function. Bottom line, if you're repeatedly getting this error, you should run an anti-spyware utility (such as SpyZooka), first, and then check your browsers for plug-ins (such as Google rankings tools, etc.) and other software that might be accessing the Internet and querying Google.