Sorry again to all my RSS-subscribers for flooding your friends pages with old content. It was pointed out to me that some other (non-LJ) RSS software was choking on my feed, and feedvalidator.org said it was not valid. I made some changes so now it is approved as valid, and of course LiveJournal then decided that there were 20 brand new entries or whatever.
However, I have an idea that I think will avoid recurrence of not only this kind of flooding, but also the occasional problem of a single old post popping back up. Which is to check the user agent string, and when it's LiveJournal requesting, I will serve only the most recent five entries, instead of the most recent twenty. I'll probably implement that change shortly, and with any luck no one will notice.
(Once again, if anyone happens to have friended me but not my blog's RSS feed, you can friend the feed here...)