Google wants to know you're human
Other people have seen this block before, though we don't hear about this too often. If you manage to set off Google's filter, it should be a pretty innocuous block. Enter in the text, and you should be back up and googling in no time.
That is, unless you are vision-impaired, as this blogger ranted in 2005. In that case, you need an alternate route around the speed bump. Google has one. Click the wheelchair icon on this page. It will read you a series of numbers, overlaid on a background of jibberish. Presumably, a human will be able to pick out the numbers, but a machine will not. This feature does not appear to be active on the block page Wilson stumbled upon.