It's known that Google takes preventative measures to reduce automated use of their search engine. In fact, Googles terms of service restrict the use of automated queries. Normally human users with real browsers will not be suspect of such use and thereby should not trigger firewall rules that detect queries that appear to be automated.
However I found myself in just that position. After running several varied queries, I came back to running the repeating a past query (through the browser drop down query history) and received the following:
Interesting to note is that the page response header is a 503 error code.
I suspect this was triggered by my complex query, retrieving multiple pages or results, and repeated usage in a short period. Google knowledge base on this topic suggests that users that have this problem may also have a virus or other spyware on their computer or another in the network.