When the AP screws up, or when you screw up by putting the wrong inputs into the FCU and/or FMGC?
That's why I put it in quotes in my first post
Regarding handflying in IMC, I'm happy to do it raw data down to about 500ft agl. If the ceiling is below that, I prefer to have the AP do it so I can monitor the big picture more effectively. In VMC, I try to go down to mins without looking outside once or twice a month.