Whoa, this is certainly interesting, google blocks "spammy" queries http://seoblackhat.com/2006/11/30/google-and-spammy-requests/
Yeah I got that thing once too, and it happened when I changed the result page from 10 to 100, this really sucks as I was just checking backlinks or something else I don't remember. That's really sad since google can no more distinguish between automated and human behaviour!
I just got one of those recently. I did an inurl: query combined with 100 results per page and I was locked out. google's absolute horrid for webmasters.