A good source of potential bugs that need fixing is error logs. Maybe one day we'll have them empty, but right now on the live cluster there is still a significant amount of small and large errors that go by. No super critical ones but ones that should be fixed and it's not rare for them to hide a bigger mystery waiting to be unlocked.
I'd say we can give this another shot at this Hackathon. Someone with shell access can perform a few searches in the major logs (apache error log, php error log, mysql error log? - on fenari?), group them by unique patterns and sort by frequency and dump it somewhere on a whiteboard. And see if we can get some fixed :)
This will affect both MediaWiki core as well as deployed extensions.