I tried from finding it myself, and I've only seen it once. I was logging into Dropbox via Chrome on my Android phone, and I must have done something to trigger captcha spambot hell. I just tried triggering it again and was not able to do so.

The ball had an animal in it and I was asked to bounce the ball, causing it to rotate. I had to bounce the ball with just enough force to get it to land so the animal was positioned upright. After several failed attempts, I gave up.