Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

The stock market ended its run of record highs today, as both the Dow and S&P pulled back slightly even as the Nasdaq Composite (NASDAQINDEX:^IXIC) rose to its highest level since 2000. Although the losses in the broader market were subdued, several stocks posted large declines, including Renren (NYSE:RENN), Fortinet (NASDAQ:FTNT), and Net 1 UEPS Technologies (NASDAQ:UEPS). Let's look at what sent these stocks plunging today.

Renren dropped 11% after reporting a lackluster earnings report late Wednesday. Although the Chinese Internet company's daily deals site posted a 36% rise in revenue, a decline in social-networking revenue produced an overall sales drop of 6%. Although Renren's loss was narrower than investors had expected, it's still a negative sign that the social-network company hasn't been able to make much progress in improving its prospects, with unimpressive guidance also pointing toward further revenue weakness in the future.

Fortinet fell 13% in the wake of its chief financial officer's departure after a disturbingly short tenure at the network-security company. The question facing investors is whether CFO Ahmed Rubaie's statement that he was resigning for personal reasons is the only reason for the move, or whether problems with Fortinet are lurking in the shadows. At this point, there's no reason to buy into conspiracy theories, but investors in other companies have gotten burned in similar situations, and that likely explains the sell-off today.

Net 1 UEPS crashed 29% after the South African Constitutional Court set aside an appeals-court decision and declared that the process by which the payment-services company got a contract with the South African Social Security Agency was invalid on a constitutional basis. The ruling is a huge setback for Net 1 UEPS, but it's not fatal, as the court will have a hearing in February to determine what it calls a "just and equitable remedy" concerning the situation. Investors hate uncertainty, but they'll have to wait until February to find out how big a problem this proves to be for Net 1 UEPS.