Groupon Inc. is revising financial results it reported in its first quarter as a public company, after discovering executives failed to set aside enough money for customer refunds.

The popular online-coupon company's auditor, Ernst & Young, called the error a "material weakness in its internal controls" for 2011. Groupon shares, which have been trading below their $20 initial public offering price, sank 6% to $17.20 in after-hours trading.

The...