Find out why Groupon's stock took a hit today, and how the coupon marketer plans to overcome its challenges.Shares of Groupon (GRPN -11.65%) fell as much as 16.8% on Wednesday morning, hamstrung by a mixed second-quarter report with gloomy guidance for the next period.Groupon lowers full-year guidance amid tech issuesGroupon reported second-quarter revenue of $124.6 million, surpassing analyst expectations of $121.7 million. The bottom-line loss of $0.02 per share came in below Wall Street's consensus targe ...