Strong SUV sales in Europe weren't enough to reassure traders after Ford's profit missed estimates. Shares fell sharply on Thursday. Image source: Ford Motor Company.

What: Shares of Ford Motor Company (F 1.12%) dropped sharply on Thursday. Ford's second-quarter profit missed Wall Street estimates, and the company warned that its full-year guidance is at risk. Shares closed at $12.70, down 8.2%.

So what: Ford's net profit was down 9% from its (extremely good) year-ago result. Ford said that rising incentives in the U.S. and some weakness in China were to blame, and that it was able to offset some of the drop with cost-cutting and a good result in Europe.

But traders quickly decided that Ford's earnings miss was a hint that the long streak of growth in the U.S. new-car market might finally be stalling. That would be bad news for the whole auto industry, especially Ford and its Detroit rivals. Ford's stock took the worst hit, but the others weren't spared: General Motors (GM 0.95%) shares dropped 3.2% despite a far more upbeat earnings report just last week, while Fiat Chrysler Automobiles' (FCAU) stock fell nearly 5%.

Now what: CEO Mark Fields has continued a practice started by his predecessor, Alan Mulally: When Ford's management team sees signs of danger on the horizon, they tell investors -- usually in conjunction with an earnings report. That's what happened on Thursday, and speaking as a Ford shareholder, I think it's a commendable practice. But it means that Ford occasionally surprises investors on earnings days, and a fast drop in Ford's stock price is often the result.

I've said on past occasions that these moments can be good buying opportunities. Today's drop might be another.