OK, this is weird.
I've set up a fresh install of Drupal 5.0. Currently it isn't running anything other than the default modules that come with the package.
I've been going through the rather laborious process of moving all of my posts from my old site (which had been running WordPress) into Drupal. I've been doing this by exporting my wordpress posts as text and then cutting and pasting them into the story module one by one. Wheee.
Out of some 30 or so articles, two of them generated 404 errors when I tried to publish them -- the others had no difficulties whatsoever. I decided to figure out exactly what was triggering the 404, so I took one of the posts and I entered just the title, category, and time stamp, putting the words "placeholder text" in the body.
That worked.
I then began to edit the document, slowly replacing the placeholder text with text from the actual article... two paragraphs at a time.
The first few cut & paste's worked just fine. Six paragraphs in, however, I received a 404. This was the text I was trying to post: