I work on the ramp on A320 and A330.
I have been sitting here for a while trying to think of any that I had seen.
All I can think of are due to other systems failing. Like Primary GPS, or the other FMC, or alignment problems with the IRUs.
The only one I can think of that is only FMS is Nav Data base out of date!
and of course when it doesn't work at all.
As a tech the FMS either works or it doesn't. Its a box of tricks. If a CB reset doesn't do it then it's in the MEL, and you either use the other one, or you LOW (look out of the window!)