Buffett bought the stock in '66. Bought $4mm, sold it for $6mm a year later. Disney compounded at around 18.9% from '67 to '95. Berkshire compounded at 24.8% during the same period. Dividends would close the gap a bit, but not much.
The investment in '66 had so many ways to win. Theme park business was set to explode. Monetization of the film library. The film business actually was not a good business post the Paramount consent decrees in '48. Disney had a niche in family focused films that allowed them to earn supernormal returns/margins. Animation capabilities no one really had. So cheap. ~4x EBIT. Mary Poppins just came out. But it was still going around the globe in '67. '67 earnings were actually up vs. '66.
Walt passed away in '66. You lost a driving force of the film business and strategic force of the company. The 70s and 80s weren't great decades. You needed Roy Disney (Walt's nephew) to replace management and bring in Eisner to turn the company around in the 80s.
Look at the current company. Massive amounts of value has been created due to ABC and ESPN. ESPN in particular has driven enormous valuation creation. How can one Buffett anticipate that?
I don't think selling was an error at all. My view is Walt drove the film franchise when Buffett bought the stock (he had a meeting with him in California, he made a judgement on Walt). His passing was a huge blow to the upside of the company. You had film in the library you could re-release or sell to TV that was probably worth the entire market cap of the company. You had the theme parks. But underwriting the film franchise growing post Walt was very hard. Forecasting theme park growth was very hard. You knew earnings were going up, but how fast? The price gave you a huge margin of safety for most of these. But it made sense Buffett sold after the pop + Walt's death.