Nothing lasts forever. There's no such thing as a free lunch. If it sounds too good to be true, it is. If it can't keep going, it won't. Caveat emptor.

Call it what you will. Berkshire Hathaway (NYSE:BRK-A) (NYSE:BRK-B) did something it hasn't done since Warren Buffett took the helm in the mid-60s: It had a terrible year.

Had to happen eventually
OK, maybe not terrible, but its worst ever by far. Per-share book value fell 9.6% in 2008, marking only the second time since 1965 it recorded negative results.

Berkshire's share performance, unfortunately, has been markedly worse. Shares are off 45% in the past year, falling back to 1998 levels.

Such is life. Most stocks are on a similar trajectory these days. Factor in investments in battered financials like Wells Fargo (NYSE:WFC), American Express (NYSE:AXP), and Bank of America (NYSE:BAC), not to mention its dozens of operating subsidiaries waist-deep in industries like construction and retail, and a 45% plunge probably seems justified.

That, however, is hardly the main driver behind the recent nosedive. The growing fear is not necessarily over Berkshire's core holdings; above all, the worry is over a set of equity derivatives that, thought goes, could ultimately implode Buffett's track record of domination.

No one's perfect
Over the past few years, Berkshire sold a boatload of derivatives that effectively bet on the long-term strength of global stock market indices. By selling equity put options, it agreed to pay huge sums of money if the indices fell below levels they were initially struck at when the contracts expire -- which fall at various times between 2019 and 2028.

Since most of the options were sold in the glory days of yesteryear-- when indices were as much as twice as high as they are today -- Berkshire's current mark-to-market loss sits at $10 billion and grows by the day -- as global markets disintegrate.

Here's where the doomsayers come in: If indices fall to zero on and between 2019 and 2028, Berkshire could be on the hook for as much as $37.1 billion. That, even by Berkshire standards, would be a catastrophic loss.

It's all a bunch of ballyhoo
Nothing seems impossible after these last 12 months. Talk about stock markets becoming worthless, and rational people take you seriously. Still, let's be real: If major stock indices go to zero, Berkshire's liabilities won't be on anyone's mind. Guns, ammunition, canned food, and bomb shelters will be.

More importantly, the uproar ignores the most important part of the derivative contracts: that Berkshire received $4.9 billion up front, and won't be liable for a penny until they expire between 2019 and 2028. I know: $4.9 billion is peanuts compared to a potential $37.1 billion liability, but it's still a lot of money, and 10-20 years is a long time.

Long enough, in fact, that even if the derivatives do ultimately end up in the red, the future value of the $4.9 billion would likely offset even a massive loss. Using a 15-year timeframe (roughly the average time from now until 2019-2028), here's the pre-tax compounded value of $4.9 billion under different returns:

Annual Return

Value of $4.9 billion After 15 years

8%

$15.5 billion

10%

$20.5 billion

12%

$26.8 billion

15%

$39.9 billion

20%

$75.5 billion

And those returns are hardly out of reach. Since last fall, Buffett has put tens of billions of dollars to work in companies like Goldman Sachs (NYSE:GS) and General Electric (NYSE:GE), with 10% yields and equity kickers offering ample upside. Just a month ago, Harley-Davidson happily sold Berkshire debt yielding 15%. When calamitous assumptions leave Berkshire with a $37 billion loss, and reasonable assumptions leave it with an offsetting $15 billion-$40 billion gain, the outcome of the derivatives -- even after last year's spectacular plunge -- seems like a far cry from lethal.

Bottom line
Make no mistake: Berkshire's hurting along with the rest of the economy. No one's claiming Buffet's 100% immune as the rest of the world flounders. Even so -- while less lucrative than they used to be -- the derivatives still embody what Buffett does best: Invest when the odds are in his favor. Keep a long-term perspective. Ignore short-term fluctuations. Repeat until wealthy.

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Fool contributor Morgan Housel owns shares of Berkshire Hathaway. Berkshire Hathaway and American Express are Motley Fool Inside Value recommendations. Berkshire Hathaway is a Motley Fool Stock Advisor selection. The Fool owns shares of Berkshire Hathaway and American Express. The Motley Fool is investors writing for investors.