To some degree it can't be helped. Accidents will happen, but I firmly believe that the right response to this is to minimise the chances, and delight with customer service.

Make sure it happens as infrequently as possible, and when it does happen make sure compensate the user accordingly.

As I say, accidents will happen, but they shouldn't happen twice to the same person, and that person should be amazed as to how far you will go above and beyond to set things right.