You may not depart on a flight, or flight pairing, while you know you'll be outside of FDP AND having the means at your disposal NOT to exceed FDP (home base have standby/replacement crew).
You may complete a duty and exceed and/or extend FDP from an outstation towards a base where replacement crew is available.
The intention of the law is to have legal planning AND legal operation.
You cannot start a duty that's not planned to be legal, AND you cannot complete a legally planed duty if the duty, for whatever reason, becomes illegal.
Who will answer the questions after the incident?
The company will say that they will NEVER let you fly outside of FDP, or tired, or sick!!
It was YOU that made the decision, you take the blame.