If you had a custom usercontrol on the page either in a custom feature or in layout.master, if it fails to load the user control it can give a page not found error.
When pages are renamed, a 301 redirect is created from the old url to the new one and it can cause a problem if you then want to re-use the old url for a new page. You should look under Administration > Advanced Tools > Url Manager, and under Administration > Advanced Tools > 301 Redirect Manager for unwanted redirects and url mappings.
Hope it helps,
Joe