In Patrick case's, YES the depositors should have been VERY wary of the sub argument as it was fully disclosed because it was dubious that his claim is true. But it is not a common mistake because the depositor for Patrick or you with your doctor neither made any claim to the effect that what the other party claimed was factual and a requirement for the contract. They have to make the same claim.
They don't have to make the same claim, they just have to both make the same mistake and that mistake has to be such that neither party would have agreed to the contract but for the mistake. In any event, in this specific case, MP did make the same claim. She said, to Patrick, "well that works".
The people who gave Patrick his funds made a risk misjudgement when they got convinced by his arguments. They did not make a common mistake.
If it's precisely the same risk misjudgment Patrick himself made, it's a common mistake.
I am not stating that this situation is fair for Patrick
Why would you knowingly choose an inequitable result when you don't have to? Why not choose a result that's fair for Patrick too? What is forcing you to knowingly be unfair? We're not a court of law (or the contract would be void for a dozen reasons, starting with usury) -- we're a pure system of equity and fairness.