William the Bastard: 1038 to 1047
You have decided to ask your cousin king Edward for military aid. But the king of England's army is not his to command outside of England: he has maybe a thousand housecarles and a few royal thegns, but no sizeable force to break through a siege. After ravaging the coastal possessions of a few of your enemies, the English return home. The siege of Rouen becomes straitened and hunger weakens the garrison. When the assault comes you manage to retreat to the castle with a few dozen men.
King Henry arrives too late, to find you either dead or in irons. Your career as duke of Normandy is at an end almost before it began.