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.

Stocks continued their run of solid performance, with further gains to record highs for broad-market benchmarks like the S&P 500 and the Dow Jones Industrials. Yet despite signs of improving economic conditions in the U.S. and globally, several stocks posted sharp declines, including Bio-Reference Laboratories (UNKNOWN:BRLI.DL), Tilly's (NYSE:TLYS), and 58.com (NYSE:WUBA). Let's take a closer look at these three companies to see why their stocks fell so far today.

Bio-Reference Laboratories dropped 22% after warning that its fourth-quarter earnings would come in much weaker than investors had expected. Even though the provider of clinical laboratory testing services gave improved guidance on the revenue front, Bio-Reference Labs blamed pressure on Medicare reimbursement rates for its expected earnings shortfall. With the likelihood of continued downward pressure on amounts the company gets from government health-care programs, Bio-Reference Labs could see further problems in the short run. Looking further into the future, though, the company has huge potential to serve increasingly customized health-care needs for an aging population.

Tilly's plunged 24% as the retailer reported a disappointing same-store sales drop of 2.4%. Although the company maintains that it should be able to improve its comps in the holiday quarter, investors clearly aren't convinced that Tilly's will succeed in defeating tough competition, especially in light of overall declines in revenue, net income, and gross margins during its fiscal third quarter. In particular, as clothing and accessories become more of a high-margin item for larger-scale retailers, Tilly's could see more pressure on its business.

Chinese online company 58.com fell 10% as even strong results weren't enough to satisfy highly optimistic investors. Gains of more than 70% in overall revenue and paying merchant-member counts for its online classified service were impressive, and guidance for future sales growth to slow only slightly from current levels seemed bullish. But the drop shows just how important it is to consider valuation in weighing earnings results, as a huge jump in 58.com's share price just since its IPO last month implied that shareholders wanted to see more ambitious expectations from the company.

Fool contributor Dan Caplinger and The Motley Fool have no position in any of the stocks mentioned. 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.