Apple Computer

(AAPL) - Get Report

Chief Executive Steve Jobs called it a "speed bump," but to shareholders of some

Janus

funds, Apple's underwhelming sales this quarter will probably feel more like a real doozy of a pothole.

After Thursday's market close, the Cupertino, Calif.-based computer maker said that on Oct. 18 it expects to announce earnings that are well below Wall Street analysts' expectations due to sagging sales worldwide, weak demand from schools and lackluster demand for its new

Power Mac G4 Cube

. The stock was clobbered in after-hours trading, losing more than 40% of its value.

Janus Capital

, manager of the top-selling eponymous funds, stands to lose more of its shareholders' money on this slide than any other fund company, according to the latest holdings information through the end of June.

The $300 billion Denver fund-shop's managers were far and away the largest holder of Apple shares at the end of the second quarter, according to

bigdough.com

, a Web site that tracks institutional stock ownership. On a firmwide basis, Apple was Janus Capital's 44th largest investment out of more than 621 holdings. Unlike some of Apple's other big investors, Janus upped its stake in the second quarter.

It's easy to see why Janus was interested. In 1998, Apple rolled out its eye-catching and ballyhooed

iMac

. Since then the stock, which had been listless, rocketed north. In 1998 and 1999, it posted triple-digit gains. Today more than 360 mutual funds own Apple shares, including about one-in-10 large-cap growth funds.

This year, however, investors had turned a bit skittish. Before Friday's inevitable bloodletting, the stock was down nearly 3% this year.

And it will most certainly go far lower. News of this shortfall is particularly disturbing because, unlike chipmaker

Intel

(INTC) - Get Report

, which also warned of weak results, Apple's woes weren't limited to slipping demand in Europe where the tanking euro is easy to blame. Key competitors such as

Dell

(DELL) - Get Report

,

Gateway

(GTW)

and

Hewlett-Packard

(HWP)

all say they're on target to match analysts' expectations.

While one cratering stock can't tank a large fund on its own, it can take a bite out of returns if more than 2% to 5% of a fund is sunk into it. Of the 10 funds with the biggest bet on Apple, three are managed by Janus:

(JASSX)

Janus Special Situations (7.6%),

(JCGAX)

ASAF Janus Capital Growth (3.7%) and

(JSVAX) - Get Report

Janus Strategic Value (3%).

Of course, many of these funds may have raised or reduced their stake since their most recent portfolio report. But Janus managers' move to buy more shares in the second quarter indicates that they didn't necessarily see this coming. While the firm's flagship

(JANSX)

Janus fund didn't make the list of funds with a big bet on Apple, it owned nearly 1% of the company at the end of the first quarter -- the latest portfolio data available.

It seems other managers might have seen some warning signs. In the second quarter, the number of institutional managers dumping their Apple stake outnumbered those initiating one, 46 to 34, according to bigdough.com.