http://tenchan4v5bnlu3gatyjyctlsb25asvncbz4pr7gmomhsrvyx5zfpcad.onion/res/142.html
So if you have opened a bunch of tabs, the last one you opened will be the only valid captcha. (You can type your valid captcha answer into any of the previous tabs and it will work.) If you do get an incorrect captcha, hit back, ctrl-c your message to avoid losing it, and refresh the page and that captcha should work correctly.