On a number of Stack Exchange sites (including meta.stackexchange.com and rpg.stackexchange.com) I'm getting stuck in a loop proving I'm a human when I try to issue a search.
I click the box (a text-based CAPTCHA may or may not appear), the blue circle spins, the green tick appears, and then I'm redirected... to the same human verification page.
Other Stack Exchange sites seem to work fine (stackoverflow.com, for example).