This list is by no means exhaustive, but is a useful guide to the pitfalls and problems that can arise with promotions:

• Cyclical promotions - you should avoid cases where an item can appear on both sides of promotions, as you can have situations where buying one item earns you another for free, and having 'bought' that item then earns you another item for free, and so on. Remember that items can be included in promotions through being in a certain category, as well as by being specifically included.
  
  
• Promotions involving items with different prices - for example, let's say you have a buy one get one free promotion on a product that has four differently priced versions. If a user picks two items, you would typically want to give the cheaper one free. However, if they buy 4 items together, and you just give the 2 cheapest free, it may cost them more than if they made two separate purchases of 2 items (where they could get the cheapest and second most expensive items for free instead of the two cheapest). The logic of dealing with such promotions in the fairest way possible is highly complex and dependent on a number of factors; what seems fair in one situation may well not seem fair in another. It is far simpler to have buy one get one free type promotions only on items of the same price.