It seems that the adult verification system caused an unexpected side effect. In order to remember which diary you were visiting when you hit a page that needs an adult ID, we store the diary name in a cookie.
Unfortunately, by default, PHP (which the new secure comments system is written in) reads cookies and automatically assigns variable names to them, as one of the last places it takes information from. I used the keyword diary in the cookie. Matt used the keyword diary in the PHP script. As such, it was possible, if you started adding a comment, then went to someone elses diary before actually saving your comment, that you could end up posting the comment to a non existant diary! The fact that it was nonexistant was purely down to the fact that I saved the diary as a string and Matt used it as a number…
This is now fixed, please let us know if you still experience comment issues.