Faulty OPKs

I'm a faithful user of the Clearblue Digital Ovulation Tests (month after month after month), and I have never had an error result before. This month, from the same box of tests, I have had two. I'm a little frustrated, considering a box of 20 here costs over 35 dollars. I know I'm using the tests correctly, and it seems odd to me to have two errors when I have never had an error before in about a year of using them. I would appreciate some feedback as to whether there was a bad batch out there or something, and I would like a coupon. As I said, they are not cheap and to lose two out of a box is pretty annoying!