"Are you sure it's not something on the server?"
"Can't see how it could be. Maybe it's some Drupal security thing that's being triggered."
Lara's blog post wouldn't save, and no error was being thrown. When she tried to save it, it just reloaded the edit page.
You know how this goes. After insisting it couldn't be the server, and that nothing had changed, I remembered that a few days ago I had enabled Apache's mod_security module.