I once had the altimeter knob come off on approach to Oxford - I got the ATIS, found I couldn't set the QNH (I'd been on regional pressure so quite a few mB different), switched to Approach, informed them, and could hear quite clearly even from 10 miles or so out the scratching of heads as they tried to work out the circuit indicated altitude I should fly to put me at 1200ft QNH.
To be fair to them, it only took them about 30 seconds to come back with an answer which I think was correct (seemed to work, anyway!).