Sometimes when I am charged for something that is meant to be included in my rate, or at full rate when I am entitled to a discount, I wonder if these mistakes really are simple oversights. Another possibility is that employees have an incentive to "mistakenly" charge guests more, hoping that it will go unnoticed. In a restaurant or spa, service charge is usually a percentage of the total bill, so there could be personal gain if a guest paid full instead of half price.

I always give people the benefit of the doubt, but am very careful to check that discounts and inclusions are correctly applied.