There's never a shortage of losers in the stock market. Let's take a closer look at five of this past week's biggest sinkers.

Company

Jan. 10

Weekly Loss

YRC Worldwide (NASDAQ:YRCW)

$13.58

29%

J.C. Penney (NYSE:JCP)

$7.34

16%

Ambarella (NASDAQ:AMBA)

$29.31

14%

Merge Healthcare (UNKNOWN:MRGE.DL)

$2.22

14%

Molycorp (NASDAQOTH:MCPIQ)

$5.42

10%

Source: Barron's.

Let's start with YRC Worldwide. The truck transporter shifted into reverse after its unionized workers rebuffed a new contract offer. It's a hairy situation, since YRC had agreed with its creditors to reduce its debt by $300 million if the union ratified the contract extension.

J.C. Penney shareholders hit the register after the struggling retailer announced that it was merely "pleased" with its holiday performance. The department-store operator did stick to its outlook calling for positive comps for the current quarter, but that still doesn't say much about its present state, given November's double-digit percentage gain.

Ambarella hit an all-time high on Tuesday, but then the pros started getting afraid of the heights. Needham downgraded the maker of system-on-a-chip video solutions on Wednesday. Deutsche Bank followed with a downgrade of its own two days later. Both analysts went from bullish "buy" ratings to neutral "hold" calls.

Merge Healthcare called in sick after the provider of software solutions for the health-care industry revised its subscription backlog lower. A former employee inflated the number of contracts. The move won't require restating prior financials, though naturally the near term won't be as rosy as originally announced, now that these bogus contracts won't be generating actual revenue.

Finally we have Molycorp taking a hit. Shares of the rare-earth materials specialist fell in each of the week's first four trading days. There was no Molycorp-specific news to trigger the apathy. Investors were merely taking profits after seeing the stock soar a week earlier, following a DA Davidson analyst upgrade.