Groupon

In behavioral economics, one of the "anomalies" from the efficient market hypothesis is the accrual anomaly.  This  anomaly states that companies with high accruals underperform companies with low accruals (so one could outperform the market by building a portfolio of long low accruals stocks and short high accruals stocks).  Accruals are broadly defined as the amount of earnings of a company that is due to accounting manifestation (e.g. accounts receivable) versus actual cash flow.  The underperformance comes when the earnings are inevitably written down during a restatement leading to a 10-12% loss on average.

Groupon is a company that books revenue on every coupon sold immediately before redemption of said coupon by the customer.  Guess what happened (a few days ago) when the company found out that the rate of redemption was lower than expected?  A classic example of this anomaly in action.