In reply to RT Cunningham.

It's not just someone turning them off, and be something out of their control, like the page didn't fully load, and so neither did your JS. In that case the form gets submitted are per normal, but no JS ran and so the comment gets picked up as spam.

It took me a LONGGG time to realise that's why I was getting so many false positives with the Spam Destroyer plugin.

And if the site has a JS bug, it can cause all comments to get picked up as spam.