I lost few campaigns.

Shortest campaign (13 turns): Early Danish.

Relying on the early alliance with the HRE ten years into the campaign, I sent most of the troops to take Sweden. Took quite a few casualties to beat those 6 units of Vikings. The very next turn, The HRE broke the treaty and invade Denmark, which was defended by 2 Vikings, one Round Spear and 1 Royal Knight. Not a chance facing a full stack. The next turn, there was a rebel in Sweden and the HRE invasion at the same time. I'm done.

Second Shortest Campaign (around 20 turns): Early Spanish

Around the tenth turn, I invade the rebel Valencia and won quite easily. The next turn, the Almohad invade Valencia. I lost the battle because I have no defense against the AUM. The King's influence drop (because he was a good runner and cowardly). In the next few turns, the Almohad managed to bribe all of my generals (including the heirs) and besiege the Spanish King in Castile. I quit before the humiliate lost.

There are few more hopeless situations where I accepted the lost and start a new campaign.