We’re looking into reports of “word verification” images (i.e. CAPTCHAs) not appearing on, for example, the comments page. If you come across a missing word verification image, the best workaround is to leave the answer field blank and submit your comment anyway. The page will reload — preserving your comment — with a new image that you can then solve and re-submit your comment.
Update, Saturday 11:45AM: We’ve tracked down one culprit for this which may be mis-matched session cookies due to a recent Blogger configuration change. The solution, at least for this case, is to close and re-open your browser.