A common mistake requires both parties to make a factual claim to be considered a common mistake. Not one party making a claim and the other accepting the claim or accepting the arguments for that claim as convincing.

I gave an excellent example:
Buyer: I need cherries delivered in Washington. Can you deliver in Washington?
Supplier: Yes, I deliver anywhere in Washington.
Buyer: Perfect. *Pay's right away*
*Supplier imports cherries right away.*
Supplier: I have your cherries ready. What is your address?

Upon which the seller and the buyer realize the supplier is in the state of Washington and can only deliver there, and that the buyer lives in Washington D.C. and that the cherries would be impossible to deliver as planned considering the supplier's means to deliver which do not currently extend outside of the state of Washington.

There is common mistake when both parties made the claim to each other and each parties accepted it from each other, but the claim is not precise enough or is false for both. There is a common claim but also a common mistake as to what that premise making the contract executable means and it renders the contract impossible to execute as a result. There is no such common mistake in Patrick's case.

Suppose this:
A borrower wants to loan 20000$ at 14% yearly interest. The lender asks him why he should grant him the loan. The borrower arguments, without making false statements, that:
 - I have a good credit rating. (true)
 - I have a stable job. (true)
 - I have enough assets to cover the loan should problems arise. (true)

The lender answers by "fair enough, works for me, I'll grant you the loan". Yet the arguments do not make the clause that the borrower will repay true.

The borrower default on his debt and insist you share the loss on the premise that there is a common mistake because he did not lie and both expected his arguments to be true which they were and deemed sufficient to grant the loan/contract and that now the loss is irrecoverable, so the loss is to be shared. The borrower keep his current lifestyle and doesn't depart with his non-essential possession to pay his debt just as Patrick did although he said he could cover the deposit.


You would be able to void full responsibility on any contract's clauses if you ever made any argument that was true yet didn't guarantee the clauses because the other party was convinced by those arguments. (The other party never said those arguments were required to be true to execute the contract or were what guaranteed the promise made by the borrower. They merely agree to be convinced by those arguments.)

If the borrower gave an extremely ridiculous argument that he could repay because "His dog is brown (True)", you wouldn't have made a common mistake. You would have put yourself in the situation. You would have been incredibly stupid to be convinced by such a statement. But you did not made a common mistake as per contract law. The borrower is fully obligated to repay the loan because that's what he claimed he would do repay it and neither party added a sub-clause that this repayment was directly dependent on "His dog being brown" making "Repayment of the loan" possible.