Forcing a captcha challenge appending
?captcha=1 to any URL, then clicking in the box "I'm not a robot", seeing a visual challenge (this is common for me, for some reason), submit... err... why I'm seeing the captcha again? Try to verify again? Click the box "I'm not a robot", see the visual challenge, fill the text, ok, then hello "An error occurred submitting the CAPTCHA" while the box itself is a-OK:
If I leave it for some time, the session expires and trying to validate I'm human returns "Google says: Error: Invalid reload parameters.".