Is That User Error?

We emailed our subscriber base saying there was a sale and to use the coupon code. When you click the actual button, it had the full price and then you had to put the code in. We actually decided to refund the difference to those 11 people. But now I can sleep at night because I felt a little bit, you know, one, maybe you could say, oh, random case, but 11 out of 73? That's pretty significant to me.