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

Nov. 29

Weekly Loss

Net 1 UEPS (NASDAQ:UEPS)

$8.19

28%

Nuance Communications (NASDAQ:NUAN)

$13.52

17%

Fortinet (NASDAQ:FTNT)

$17.10

13%

OraSure (NASDAQ:OSUR)

$6.15

12%

Renren (NYSE:RENN)

$2.98

11%

Source: Barron's.

Let's start with Net 1 UEPS. The shares tumbled later in the week, after the South African Constitutional Court struck down an appeals-court decision, invalidating a contract that it had secured with the South African Social Security Agency. The appellate court expects to submit a remedy early next year. 

The market didn't want to hear what Nuance Communications was saying. The speech-recognition specialist offered up guidance for the new quarter calling for a sequential dip in revenue on an even steeper drop in profitability. Analysts weren't expecting that. 

Fortinet fell after its CFO stepped down. Despite claiming he was leaving for personal reasons, the market doesn't like uncertainty. It also doesn't help that his term was so short. Ahmed Rubaie was appointed as CFO back in April. 

OraSure slipped after a customer had a setback. The maker of oral fluid diagnostic and collection devices revealed that its customer received a letter form the FDA, suggesting that it refrain from marketing its service in this country. OraSure is still sticking to its earlier revenue guidance. The customer accounts for less than 5% of OraSure's business.

Finally we have Renren. The Chinese social-networking website operator tumbled after offering up horrendous guidance. Renren now sees revenue falling 36% to 41% in the current quarter. That's brutal. The only saving grace is that the cash-rich Renren isn't trading for much more than its cash and investments that now translates into roughly $2.80 per American depositary share.

Longtime Fool contributor Rick Munarriz has no position in any stocks mentioned. The Motley Fool recommends and owns shares of Nuance Communications. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.