"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."

Source: Of Permanent Value : The Story of Warren Buffett. Book by Andrew Kilpatrick, Vol. 2 (p. 1615), 2007.