I'm trying to diagnose a problem on a Moodle v2.5.x that's been working great with no problems for some time. Today, I got an email from the admins saying that when they click "Turn Editing On" some of the block edit icons appear briefly and then vanish. In particular the pencil icon that allow the user to edit the block title and the double-triangle icon that allows the user to "move navigation blocks" both vanish. They're visible for 1 to 2 seconds after clicking "Turn Editing On" but then vanish. They are not visible on the page after that and the related HTML does not show up when you do a "view source".
The users claim not to know what happened to cause this and I'm a bit baffled as to how to diagnose it. Is there an admin configuration page somewhere that allows one to hide some of the block editing icons? Or does this sound like any kind of known bug that could have been triggered by something a user did on the site? Any suggestions would be really helpful.