We have a "shop" section of MrThankYou.com, and I "accidentally" found out that we had received 3 orders last month but we never got notice. It's not all rainbows and sunshine over here in Mr. Thank You land. We make mistakes.
I believe taught me, "A customer service error is just an opportunity to make a customer for life." I sent each book or set of Thank you cards with a personal thank you/apology note and an extra copy of my book. Remember, "Results show skill, but how we handle problems shows character. Mistakes can be a tool for greater connection and trust.