In his annual letter to Berkshire Hathaway shareholders, Warren Buffet admitted that he personally made a mistake that cost the company $11 billion. He said that the $11 billion loss resulted from a write-down in the value of a few subsidiary and affiliate businesses that Berkshire Hathaway owns.

The final component in our GAAP figure – that ugly $11 billion write-down – is almost entirely the quantification of a mistake I made in 2016. That year, Berkshire purchased Precision Castparts (“PCC”), and I paid too much for the company.

No one misled me in any way – I was simply too optimistic about PCC’s normalized profit potential. Last year, my miscalculation was laid bare by adverse developments throughout the aerospace industry, PCC’s most important source of customers.

In purchasing PCC, Berkshire bought a fine company – the best in its business. Mark Donegan, PCC’s CEO, is a passionate manager who consistently pours the same energy into the business that he did before we purchased it. We are lucky to have him running things.

I believe I was right in concluding that PCC would, over time, earn good returns on the net tangible assets deployed in its operations. I was wrong, however, in judging the average amount of future earnings and, consequently, wrong in my calculation of the proper price to pay for the business.

PCC is far from my first error of that sort. But it’s a big one.

BERKSHIRE HATHAWAY INC. 2020 Annual Report