Well, I just had one.
I've been advertising One Stitch Designs for a while now on various fashion and fiber artists blogs. My ad has a coupon code in it, same one I've used in my Etsy shops when I have advertised before.
Well here's my facepalm moment.
I made the code in my website and went on with my life. And today when I went to add a customer specific code (for joining my mailing list), I realized it wasn't active. I forgot to triple check everything and make sure it was active!
That was smart. I wonder now if anyone had gotten to my cart and when the code didn't work, they left. I know I've done it in the past.
Moral of the story: always check everything.