A320 series
Problem: Loss of Altitude Scale on one PFD and loss of one FD (we had 2FD2 on FMA). One AP, Autothrust and CAT 3 inoperative on ECAM status. No ECAM caution/warning generated.
Solution: Check Baro Reference setting on affected side. In our case the Baro Ref had reverted to 745Hpa due to a FCU failure mode. After re-setting the QNH everything went back to normal. For some reason this was not picked up by the FWC (apparently it was an intermittent fault, after a couple of minutes we got the ECAM caution 'FCU1 fault', which kept disappearing and reappearing until we used emergency cancel).