If you use advanced search features say 10 times in 10 minutes or whatever (a reasonable amount when refining a search if you ask me), you're quite liable to be elected to have a trial of endurance against the "prove you are a human" feature, having to solve multiple (my record is 16) consecutive "select all images that contain BLAH" tests.
I had to solve thousands of captchas as part of the yahoo groups archiving project. You only have to choose four of the images, whatever the test is, and it's not really precise, so you can make small mistakes and it still will let you pass.
