A: My house is 1,200 square feet.
B: I know, I measured it. I can paint it for $2,500.
A: Agreed.
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Common mistake. If the house isn't 1,200 square feet, A and B are jointly responsible.
This is like the last case. Both parties, with sufficient information to realize otherwise, concluded that the loan portfolio was free from correlated risk. Patrick's agreement was based on that shared mistaken conclusion.
Using your example...
Patrick is assuring the person paying for the paint job that the house is 1,200 square feet after measuring it himself. The house turns out to not be 1,200 square feet. It is PATRICK'S fault that the house is not 1,200 square feet, because HE was the one in the position to ensure the house was 1,200 square feet.
It is NOT a common mistake, because those investing in Patrick had no choice of who to invest in, nor did they have any way to verify that they were not PPT's. Patrick was the ONLY one in that position, and he gave people false information. Sure, he didn't know it was false information at the time, but it is on him that the information ended up being wrong.
To revise your example:
A: I have no idea how big my neighbor's house is, but I want you to paint it.
B: I measured it, and it is 1,200 square feet. I can paint it for $2,500.
A: Agreed.
B: Whoops - turns out you owe me $5,000, because I asked the home owner how big it was, and they said it was only 1,200 square feet. Turns out it's actually 2,400.
A: No. Just, no.