I use to get it as well, until I realised that my Firefox User Agent string was edited. As soon as I restored it back to the default, I've never encountered the error since.
now try another example SEO chat google.com/search?q=site:forums.seochat.com&hl=en&start=160&sa=N you get the error message also still ??????
I have received that message a few times and its usually when I make simultaneous searches that are similar. I cant remember the search terms but I do remember that.