Mr. Buffett acknowledged that times had changed. "There have been more moats that have become susceptible to invasion than seemed to be the case earlier," he said.

To a large degree, Mr. Musk is a living embodiment of a gleeful invader. With Tesla, he is trying to prove that what was long thought of as a substantial moat around the automobile industry could be overcome. He has gone direct to consumer with Tesla vehicles, bypassing the traditional car dealers that were once considered a barrier to entering the automotive business. He even had what seemed like his own moat: a network of Supercharger stations around the country that he recently opened up to competitors. (His initial panning of the "moat" idea followed a question about why he was willing to cede the advantage that the closed charging network provided.)

But, in truth, Mr. Musk's experience — and challenges — at Tesla may demonstrate just how much a moat remains a powerful deterrent to competitors.

In the case of the automobile industry, perhaps the biggest barrier is the huge amount of capital needed — Tesla has already raised more than $12 billion. But even with huge amounts of money, Tesla's recent production struggles — it has repeatedly missed its targets for making the new mass-market Model 3 — show the immense value of manufacturing experience.

Mr. Musk's willingness to challenge convention has clearly pushed the automobile industry forward, but his progress has lately been bumpier than he initially expected. That's not to say he won't succeed, but if he does, he may be the exception, not the rule.

There are still huge swaths of our economy that have dauntingly wide moats, many of them protected by regulatory walls. Banks, which have been speculated about as ripe for disruption by financial technology companies, seem more likely to buy start-ups than compete against them. Airlines — which Mr. Buffett once called "a death trap for investors" — have consolidated, and enormous corporations control virtually all the landing slots at airports around the globe. Before it could get off the ground, any insurgent hoping to challenge them would have to find a spot to come down.

Even many digital businesses have a moat: Google and Facebook control so much of the advertising market that it is hard to see how a kid in a garage with an idea could usurp them anytime soon. Amazon has itself has become so powerful that virtually every seller feels it has to go through it. And Mr. Buffett has convinced himself that Apple's deep relationship with its customers through its ecosystem of products and iCloud mean the business has a defensible moat.

"There are some pretty good moats around," Mr. Buffett said. "Being the low-cost producer, for example, is a terribly important moat," he said, citing Geico, the low-cost insurance company Berkshire owns.

Saturday night, after the Berkshire meeting ended, Mr. Musk was still tweeting: "Saying you like 'moats' is just a nice way of saying you like oligopolies."

For better or worse, the limited competition that oligopolies face is usually good for investors — even if consumers might disagree.

But whether they have been dug by a company's experienced hand, the goodwill of its customers or the heavy machinery of governmental regulation, moats remain a formidable form of protection, even from the most willing of raiders.

Perhaps Charlie Munger, Berkshire's vice chairman, put it best: "Elon says a conventional moat is quaint. And that's true of a puddle of water."