So, comment spammers have been hitting my Serendipity installation via the trackback mechanism, with the result that they show up in the “Recent Comments” section, although their comments are not reachable via any particular post. One such spammer was doing this almost exactly once an hour, from different IPs every time, increasing the ostensible ID of the underlying post each time.
Trackbacks are pretty wide-open in s9y, so regretfully I just disabled the whole feature. Now, with trackbacks stopped and comments captcha-wrapped, I would like to make a “famous last words” pronouncement: [ahem…] there is now NO WAY for a comment spammer to get through without at least taking the trouble to solve a captcha. (Yes, spammers, that’s a challenge to make me eat my words … which I expect to do shortly.)