Joomla - 2.5 has a captcha form element built in - which is great. However, if you install K2 (2.5.6 or newer) this also uses the same ReCaptcha element. Unless you get the settings just right the two will conflict in strange ways. In order for the system to work you need to:

1. Disable the Captcha-ReCaptcha component in the Joomla Component Manager
2. Go to the User Manager and then to the Options (Users Configuration) and set Captcha to be "none selected"
3. Go to the K2 configuration and enable Captcha and enter your public and private keys.

If you don't do this, then both captcha's will be used but only one will be displayed (the K2 one I think) and you will get error messages similar to this:

"The words you typed did not match the ones displayed. Please try again."

Or if you disable just the K2 Captcha then you get this error message:

Empty solution not allowed.