I think I can pin it down to when I have had the editor session open for an extended period of time, so perhaps the lock has expired? How does that affect the saving of the page?
When the lock has expired you will get an alert. While the lock is in effect anyone trying to access the page will get a locked message and be blocked from editing. So you would know about the lock. The locktimer has no effect on a save unless (as I seem to recall) there is a conflict because someone else grabbed hold of the page after the timeout (you've gone to lunch and didn't close the page) and there is now more than one version of the page.