I surf Stack Overflow using a proxy server with Firefox 13.
What I am noticing in the past month, is that whenever I post an answer, it shows an Invalid/Bad Request error.
In this case, I click my browser's back button and then click it. It redirects to the CAPTCHA page and then my answer gets submitted.
This always happens, and sometimes while voting and commenting too. I dont know what is going through the website. It was pretty good previously.