Shares of high-end audio company Boston Acoustics (NASDAQ:BOSA) were off about 20% today following the announcement that a customer accounting for 32% of its sales for the first nine months of the current fiscal year will slash its annual orders by roughly half beginning in May.

The company's press release doesn't specify the customer. Neither, it appears, do the articles written on the topic today. Importantly, however, it doesn't appear that the culprit is Gateway (NYSE:GTW), the only customer named in Boston Acoustics' latest 10-K, filed last June.

Gateway, subject of a recent article by Rick Aristotle Munarriz and acquirer of budget PC company eMachines, does buy most of Boston Acoustics' multimedia products -- and Boston Acoustics' sales to Gateway, down significantly in recent years, were seen falling further in fiscal 2004. But management has already accounted for this in its internal planning, and its latest quarterly report says the customer in question today comes from its core business, which makes home, automotive, and other audio products.

At any rate, Boston Acoustics now expects to report an operating loss during Q1 of fiscal 2005 as a result, down from a slim operating profit last year. Management says it still plans on a profit for the full year.

Investors certainly have reason to react strongly today, especially given the company's recent run of good news. The shares had outperformed the S&P 500 over the 12 months leading up to today, a deal with Visteon (NYSE:VC) to serve DaimlerChrysler (NYSE:DCX) brings optimism, and the company is in good financial health.

More than anything else, it seems, today illustrates an important lesson. Investors have a distaste for surprises -- especially in the case of small companies that rely heavily on just a few customers for revenue.

Fool contributor Dave Marino-Nachison doesn't own any of the companies in this story. He can be reached via email.