OK, how would the computer handle this one, which actually happened. FO flying, A/P 2 engaged. Cleared to FL230, which the aircraft merrily sailed through. FO levels and returns to FL230. re-engages A/P 2 and aircraft starts to climb. Capt takes control, returns to FL230, engages A/P 1. All normal. Diving into the pages of the FMS it transpires that, although the FO's instruments show STD (1013) selected, and the altimeter agrees, the FMS 2 is stuck on QNH. Computer says "everything normal", and gaily climbs to 23,000ft.