I recently took a Groupon offer up for Sports Massage and Consultation. Speaking to the business owner about how successful it was he said that it had worked well, but that Groupon had oversold his offer. He wanted to sell around 250 and they had gone on to sell 400. Each massage and consultation takes him 1 hour. So he needs to fit in another 150 hours of work at a much lower rate than his standard fee. Plus he still needs to accommodate his regular (non-Groupon) customers.
From Groupon's point of view they don't really care. They collect £18 for each one they sold (and passed on £6). They make more money if they oversell.
Their response to him was that, "surely this was a good thing - more potential customers" (except this means 150 hours at £6 an hour), and "why not contact the 150 over subscribed customers and explain (Groupons) mixup" (i.e. he would be the source of the bad customer experience).
He'd heard of several other businesses suffering similar problems.