Sorry, you look like a bug.

So, this may be fixed in a bit, and it may be that trying to refine the bug
will cause it to not work, but... It seems Google is trying to prevent bots
and bugs from scouring the Internet, and this might bleed into normal
usage. If you search for the two words forum and topic in the
same search and go to page 19, you should be told that you're searching like
a virus. I think you can save a few clicks and just go to
http://www.google.com/search?q=forum+topic&hl=en&start=180&sa=N but
maybe not. Either way, I find it entertaining.