(Apologies for the Atlassian bug report here - I have spent at least thirty minutes going around support channels in the Atlassian system to report a bug for their benefit, and would like to have it recorded somewhere - various attempts to report this bug have been prevented by permission issues and other frustrating hurdles).
I recently received an invitation to join a Bitbucket group, and was having trouble getting past the second screen (name/email/username was fine), it kept saying that something was incorrect. I assume, in retrospect, that it was wanting me to complete a CAPTCHA that had not rendered.
I therefore tried signing up for an account first, so I could then accept the invitation without further trouble. A screen (literally!) asked me if I was a robot, but provided no means to prove I am not. It seems that Firefox 47.0/Ubuntu 14.04 does not render the (Google) CAPTCHA device.
I switched temporarily to Chromium and the CAPTCHA device rendered fine. It looks like there is a JS issue either with the Atlassian sign-in system, or perhaps upstream at Google. Initially it looked like NoScript might be at fault, so I disabled this, and the issue persisted.
Should community members know of an easy way to report this to Atlassian in a more correct channel I am all ears, but I am not willing to spend more than a couple more minutes on it!