Let me try it one more time: Say two people each, through equal fault, believe there's 1,500 pounds of cherries in a truck. They each have no doubt this is true, even though there is actually 1,200 pounds of cherries in the truck. In this context, there is no difference to them between "the cherries in the truck" and "the 1,500 pounds of cherries in the truck", because they both believe there are 1,500 pounds of cherries in the truck. Now, say one agrees to sell the [1,500 pounds of] cherries in the truck to the other for $3,000. Then, they discover there isn't really 1,500 pounds of cherries in the truck. What do you do?
One can argue that the contract said $3,000 for the cherries in the truck, so he's still owed $3,000. The other can argue that there should be 1,500 pounds of cherries in the truck, as agreed, as the other guy should add cherries to the truck.
This is a case of common mistake -- a contract premised on a shared mistaken belief without which neither party would have entered into the agreement and which is central to the agreement. In this case, you can't enforce the contract as agreed because the contract "as agreed" requires there to be 1,500 pounds of cherries in the truck.
No. It might not have been on the contract but if he claimed there was 1500 pounds verbally and there is not 1500 pounds, one could contest the contract for misrepresentation, regardless if the seller knew he was wrong or not, he made a claim and that claim should be honored. If the seller cannot honor the contract which requires 1500 pounds, he cannot either refund it partially either and share loss. Since he cannot deliver, he cannot keep/void any part of the $3000 which cannot be either given to seller as part of the contract.
Let me rephrase your example:
A company creates a product which they claim to be packaging and selling in packs of 4. Someone buys the packaging also believing there is 4 items per pack and it's shipped to them. The buyers and the company realize there was an error and only 3 items are inside each pack.
Company to buyers: "Sorry, common mistake, we both believed there was 4 products included but we couldn't deliver what we claimed and you believed it. We're just as much at fault for that now that it's shipped and we paid for packaging and delivery. Since we both made that mistake on assumption of the package's content and we can't undo those shipments, it would be fair to share the loss."
Your proposal would not hold in any jurisdiction that I know of. If they made a claim to deliver 1500 pounds for 3000$, they would have to deliver what was claimed or otherwise refund the money which was given based on the claim they would receive 1500 pounds which didn't occur. If someone doesn't deliver upon his claims, he's always at fault for making claims he could not deliver on, not the ones believing their claims. Although maybe you simply do not agree with the common practices. Which would be fair enough, everyone is free to have their opinion.