Here's a trivial example: Both parties to an agreement believe a truck contains 5,000 pounds of cherries and both believe that $2/pound is a fair price. They agree to sell the cherries for $10,000, based on their common correct belief that cherries are worth $2/pound and their common mistake belief that the truck contains 5,000 pounds of cherries. If it turns out the scale was broken and the cherries actually weigh 4,500 pounds, how much is "his debt"?
The argument would be that you can't look to the contract because the contract doesn't say what happens if the cherries weigh 4,500 pounds. Everything written in the contract is based on the assumption that the cherries weigh 5,000 pounds. (Unless it contains some clause about the weight, of course.) Here, it is clearly unjust to enforce the contract as agreed because the agreement was predicated on the shared belief.
Assuming you mean a buyer and a seller by "both parties," the number of cherries wouldn't make a bit of difference unless mentioned in the contract. If the buyer didn't do his due diligence in verifying the number of cherries on the truck, and didn't add wording specific to the number of cherries he was receiving, that was his problem.
In the case with Patrick, the number of cherries WAS specified. Well, the interest rate was, anyway. And Patrick failed to hold up to that interest rate. It'd be like the seller of the cherries writing in the contract that he was selling 5000 pounds of cherries, but he only brings 4500 pounds, using the excuse that someone must have stolen the other 500 pounds out of the back of the truck last night. That doesn't make a bit of difference - the buyer bought 5000 pounds of cherries, not 4500, and unless a new contract can be established (likely with a reduction in price), the seller is in the wrong, not the buyer.
You stick to the word of the contract no matter what. That is what is enforceable by law. Assumptions DO NOT MATTER.