I recently upgraded from WordPress 2.5.2 to 3.5. After doing so I found that I was unable to enter comments using Internet Explorer 9. I was able to do so with current versions of Firefox or Opera.
When entering comments under IE 9 <b>only</b> I received the message:
"Invalid data. Please try again"
When I disabled WP-Captcha-Free, I was able to enter comments via IE 9.
My blog uses the Thesis theme.