I've had a few instances where I am asked to put a CAPTCHA code or something like that in.  As we all know, sometimes the computer makes errors, and so even though I *KNOW* I entered the right code, the system kicks me to a page that tells me it was wrong.  When I go back to try again....the comment that I just typed is gone.

I think this has happened with CAPTCHA plugins but maybe others as well. 

The point is, if that happens, I'm probably not going to re-type the comment.  Instead, I'm going to probably get frustrated, swear under my breath, and leave. 

Most times, like with the 'Confirm you're not a spammer' check box, it just pops up a message that you have to acknowledge, then go take care of it, all while the comment you just spent time typing is still there.

So I would kindly suggest that you test your comment system and make sure that it's not set up so that an incorrect spam validation does not force the user to start all over.