I think that sometimes they ask you to find something that isn’t there in the expectation that you will correctly avoid ticking anything. It is difficult for me to differentiate between a car and a bus, and to work out how much of bicycle or motorcycle riders to include. I haven’t been timed out recently, and then only when the system was slow, but I have sometimes had to go through up to a dozen sets before it is satisfied. (Famous last words.)
It has been getting more frequent recently, but I mostly get it when going to Maps on Google. I have not, so far, had this on my .yahoo.co.uk e-mail.