I googled it but to no avail, so more out of curiosity than a 'need to know' but WTF causes this!!??

seen with P0300, P0301 and P0302 on an '02 Punto of all things...(turns out no.2 plug cap had blown its insulation and was arc-ing to the head, causing a horrendous misfire)

That's the "Driver has ignored the humped bridge sign" warning

I've got P1314 as Catalyst damaging misfire. Meaning huge amounts of fuel are detected in the exhaust
quote
If catalyst damage levels of misfire are detected the fault code is stored immediately. If misfire above the emission threshold is detected, then a code will be stored. The MIL will flash as long as catalyst damage misfire is present. The condition can be erased on intervening fault free driving cycles as allowed in OBD II regulations.

I have P1314 also like Catalyst damaging misfire in my list of Fiat fault codes.