There are a large class of pages in moodle which after you perform some action end up at a url which isn't a valid url, but where there is actually a url that represents that resource.

A few examples:

http://moodle.local/admin/settings.php?section=messageinbound_mailsettings

after a save becomes:

http://moodle.local/admin/settings.php

which if you try to load as a get request will fail. There are multiple situations where this manifests itself:

1) If a browser is closed and reopens with sessions retained, it will http 'get' the page (it won't attempt to re-post)
2) You are on a previously posted page, and just hit enter in the url bar to reload it (you want to load the page, not re-post)
3) You want to cut and paste the url to share with someone, or are using a url syncing browser plugin

Some web frameworks get around this by always doing a http POST followed by a redirect to GET page which can be safely reloaded. A simpler hybrid strategy is to duplicate the 'view' get parameters into the form post action, which also saves a redirect so is faster.