Looking back about eighteen months ago, Groupon had some issues with their email link encryption. When some Groupon users added "addx" into a Google search (exact Google search = allinurl: addx site:groupon.com) of the site, they were able to view older emails from other customers.
According to Tech Crunch, the problem has resurfaced again, and as it did before, the flaw is revealing email communications from some of their customs. Apparently, it is only a small number of users who are effected by the glitch, however, I think very few people enjoy having their purchase history and contact information broadcast over the internet.
Last time, Groupon director of engineering, Shinji Kuwayama confirmed, the problem came from subscribers who pasted their deals into publicly crawlable pages on the web. Either way, the emails all come from deals that ended before 2012, so it's not actually interfering with any current transactions.
No word yet on how the problem was fixed last time, or if it was even fixed. Also according to Tech Crunch, Groupon has been alerted of the problem through several different channels. Hopefully, they will act quickly to get the problem resolved. We'll keep you updated on any changes with the glitch.