Bad recommendations, good algorithm

If you've ever shopped online (*cough* Amazon *cough*), you've probably experienced the "vacuum cleaner effect". You carefully buy one expensive item (e.g. a vacuum cleaner) and then you receive dozens of recommendations for other vacuum cleaners to buy: by email, everywhere on the retailer's website, or sometimes in the ads you see on other websites. In other terms, Amazon is a 1 trillion dollar company that employs hundreds of data scientists and is incapable of understanding that if you bought an expensive appliance, buying another one of the same category in the next weeks is what you're *least* likely to do! But let's think about the problem for a second. Suggesting item that are similar to what you just bought is actually the core feature of recommendation algorithms!