If you understood a business perfectly and the future of the business, you would need very little in the way of a margin of safety. So, the more vulnerable the business is, assuming you still want to invest in it, the larger margin of safety you’d need.
If you’re driving a truck across a bridge that says it holds 10,000 pounds and you’ve got a 9,800 pound vehicle, if the bridge is 6 inches above the crevice it covers, you may feel okay, but if it’s over the Grand Canyon, you may feel you want a little larger margin of safety in terms of driving only drive a 4,000 pound truck across. It depends on the nature of the underlying risk. We don’t get the margin of safety now that we got in the 1970s.
The best thing is to learn from other guys’ mistakes. Patton used to say, “It’s an honor to die for your country; make sure the other guy gets the honor.” There are a lot of mistakes that I’ve repeated. The biggest one, the biggest category over time, is being reluctant to pay up a little for a business that I knew was really outstanding. The cost of that I think is in the billions, and I’ll probably keep making that mistake.
The mistakes are made when there are businesses you can understand and that are attractive and you don’t do something about them. I don’t worry at all about the mistakes that come about like when I met Bill Gates and didn’t buy Microsoft or something like that. Most of our mistakes have been mistakes of omission rather than commission.
Source: Berkshire Hathaway Annual Meeting