Charles Rex Arbogast/Associated Press

Groupon revised its financial results on Friday, an unexpected restatement that deepened losses for the daily deals site and once again raised questions about the accounting practices of the newly public company.

As part of the revision, Groupon disclosed a “material weakness” in its internal controls, saying it had failed to set aside enough money to cover customer refunds. The accounting issue increased the company’s losses in the fourth quarter to $64.9 million from $42.3 million.

The news sent shares of Groupon tumbling 6 percent, to $17.29, in after-hours trading. Shares of Groupon have fallen more than 30 percent since the company went public in the fall.

The disclosure highlights current concerns about the reliability of Groupon’s financial statements.

Founded only four years ago, the company has experienced astonishing growth as it came to dominate the world of daily deals. Last year, investors clamored for shares of the newly public company, which was valued as high as $20 billion.

But Groupon has suffered a number of missteps in the harsh glare of scrutiny, particularly related to its accounting.

Last summer, the company was forced to drop a widely criticized profit measure that excluded marketing costs. Several months later, Groupon restated its revenue after the Securities and Exchange Commission challenged its methodology.

On Friday, Groupon said the latest accounting problems related to certain assumptions and forecasts the company used to calculate its results. In particular, the company said it underestimated customer refunds for higher-priced offers like laser eye surgery.

Groupon collects more revenue on such deals, but they also carry a higher rate of refunds. The company honors customer refunds for the life of its coupons, so those payments can affect its financials at various times.

Groupon deducts refunds within 60 days from revenue; after that, the company has to take an additional accounting charge related to the payments.

As Groupon prepared its financial statements for 2011, its auditor, Ernst & Young, determined that the company did not accurately account for the possibility of higher refunds. By the firm’s assessment, that constituted a “material weakness.”

“We did not maintain effective controls to provide reasonable assurance that accounts were complete and accurate,” Groupon said in its annual report, which was released on Friday.

The company added that it was working with an unnamed auditing firm to eliminate the problem with its financial controls.

As part of that effort, Groupon said it had updated its refund model to reflect the new mix of deals and their more varied price points.

People close to the company said it already had accurate projections of refund rates for its more traditional deals, like discounts for restaurants and gyms.

“We believe this updated model will enable us to more accurately track and anticipate refund behavior,” the company said in a statement.

It remains to be seen whether the problem reflected the usual growing pains of a young company or a more fundamental problem with its practices.

Ray Ball, an accounting professor at the University of Chicago Booth School of Business, said the issue behind Friday’s restatement was not necessarily a major concern, given its small effect on revenue and earnings.

“They have to incur some costs here, but this is a standard start-up problem,” he said. “And they are pioneering a business model. No one has done this before.”

But Professor Ball added that the issue raised concerns about Groupon’s push into higher-priced deals like travel, and whether it would face a more regular cash drain from higher refund payments.

“If they find a lot of regrets from customers there,” he said, “that could severely affect their business model.”

On Friday, Groupon sought to assure investors that the problem was contained, saying other performance measures, like cash flow, remain unchanged.

The company also reaffirmed its earnings guidance for the first quarter of this year. It expects revenue of $510 million to $550 million, with operating income of $15 million to $35 million.

“We remain confident in the fundamentals of our business, as our performance continues to highlight the value that we provide to customers and merchants,” Jason E. Child, Groupon’s chief financial officer, said in a statement.