Hi Colin,
I've never seen this problem myself nor has anyone else ever reported it and since you cannot reproduce it, it seems likely that there was a misperception. If content authors claim it happened perhaps its because they were able to edit themselves and were thinking other users saw the same edit controls that they saw.
If there is anything like a caching proxy server in betwwen the users and the site its possible the proxy served a cached copy of a page with edit links and icons, but clicking them would not allow a user who is not in an edit role to edit even in that case.
You are running a rather old version, I would recommend you upgrade soon. A new release will be available tomorrow.
Best,
Joe