"Stupidity," Buffett said during an interview Monday on CNBC's "Squawk Box."
"I was impressed with Jeff early. I never expected he could pull off what he did ... on the scale that it happened," Buffett said Monday. "At the same time he's shaking up the whole retail world, he's also shaking up the IT world simultaneously."
"These are powerful, powerful ideas with big potential, and he's executed," Buffett said.
Read stories from CNBC's interview with Warren Buffett
Buffett defends buyout firm's 'standard capitalist formula' of layoffs
Buffett says IBM's Watson will have greatest value when it replaces human labor
Buffett says Trump hasn't had much effect on the economy yet
Warren Buffett: I used Berkshire's insights into the furniture business to figure out Apple's worth
Warren Buffett says the fake account scandal at Wells Fargo hurt its reputation
Republican health-care bill will 'cut the hell out of taxes' for the rich, says Warren Buffett
Warren Buffett says he never wants to be out of the stock market
Buffett says United made a mistake in its initial response to dragged passenger
Charlie Munger: 'The amount of waste from overtreatment of the dying is just disgusting'