Back in January, Bill Ackman’s Pershing Square Holdings sent this letter to investors:

Dear Pershing Square Investor,

Beginning on Friday and over the last several days, we acquired more than 3.1 million shares of Netflix, making us a top-20 shareholder in the company. The opportunity to acquire Netflix at an attractive valuation emerged when investors reacted negatively to the recent quarter’s subscriber growth and management’s short-term guidance.

. . . We have greatly admired Netflix both as consumers and as investors, but have never previously owned a stake in the company. Netflix is a primary beneficiary of the growth in streaming and the decline in linear TV driven by its superior customer experience, a vast and diverse amount of superb, constantly refreshed content, global improvements in bandwidth, and the proliferation and continuous improvement and convenience of devices on which one can watch.

. . . We are pleased to add Netflix to our portfolio. Many of our best investments have emerged when other investors whose time horizons are short term, discard great companies at prices that look extraordinarily attractive when one has a long-term horizon.

At the time it was seen by many as big but smart $1.1bn trade. But then Netflix reported the first decline in subscriptions in a decade.

And guess what just landed from Ackman:

Dear Pershing Square Investor:

Today, we sold our investment in Netflix, which we purchased earlier this year. The loss on our investment reduced the Pershing Square Funds’ year-to-date returns by four percentage points. Reflecting this loss, as of today’s close, the Pershing Square Funds are down approximately two per cent year-to-date.

While we have a high regard for Netflix’s management and the remarkable company they have built, in light of the enormous operating leverage inherent in the company’s business model, changes in the company’s future subscriber growth can have an outsized impact on our estimate of intrinsic value. In our original analysis, we viewed this operating leverage favorably due to our long-term growth expectations for the company.

Yesterday, in response to continued disappointing customer subscriber growth, Netflix announced that it would modify its subscription-only model to be more aggressive in going after non-paying customers, and to incorporate advertising, an approach that management estimates would take “one to two years” to implement. While we believe these business model changes are sensible, it is extremely difficult to predict their impact on the company’s long-term subscriber growth, future revenues, operating margins, and capital intensity.

We require a high degree of predictability in the businesses in which we invest due to the highly concentrated nature of our portfolio. While Netflix’s business is fundamentally simple to understand, in light of recent events, we have lost confidence in our ability to predict the company’s future prospects with a sufficient degree of certainty. Based on management’s track record, we would not be surprised to see Netflix continue to be a highly successful company and an excellent investment from its current market value. That said, we believe the dispersion of outcomes has widened to a sufficiently large extent that it is challenging for the company to meet our requirements for a core holding.

One of our learnings from past mistakes is to act promptly when we discover new information about an investment that is inconsistent with our original thesis. That is why we did so here.

By our back-of-the-envelope calculations, that’s a nearly $400m loss in a matter of weeks. Shoulda held on to that interest rate hedge!