I have found, when editing my answers via my iPhone on SO in the Full Site (Non-"Mobile" ) version, that after clicking "Save Edits" I am taken to the "Page Not Found" error page and my edits are not applied to the answer.
I have also tested this using Chrome with
open -a Google\ Chrome.app --args '--user-agent="Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A543a Safari/419.3' but I do not get the same "Page Not Found".
It works just fine when faking the user-agent via a normal browser, but it happens 100% of the time from my actual iPhone.
Seems this can happen on a Blackberry as well.