So, what's happening at Palantir Technologies Inc (NYSE:PLTR), the data giant that saw $28 billion wiped from its value in after-hours trading?
The simple answer is gravity.
When you are valued at such a sky earnings multiple (around 600 times prospective earnings, according to reports) it doesn't take a lot to puncture confidence.
Remember, the valuation of a business reflects how well investors expect it to perform - it's the market's crystal ball.
Anyway, the 10% fall after hours reflects a potential brake in the form of a Washington Post report that US defence budgets could be cut by 8% annually for the next five years.
This isn't good news for Palantir, which provides software and technology to American government agencies, including the Pentagon.
Confidence was further dented by the revelation that CEO Alex Karp is being allowed to sell nearly 10 million shares over the next six months.
This was taken as something of a warning on the prospects for Palantir.
The actual evidence is inconclusive, but director 'sell' trades are viewed as canary-in-the-coal-mine moments.
Certainly, when a company's price-to-earnings multiple is in the stratosphere even the merest whiff of cordite is enough to bring the edifice down.
The stock, which topped out recently at around $125, fell $12.56 to $112.06 in a tricky session for both Karp and Palantir.