Last time, we ended a discussion of vending machine design with a short puzzle: What problems do you see with numbering the products from 1 to 99?
I'm not saying that these are the only possible answers, but they are ones that came to mind when I thought about it.
- Product codes less than 10 would be ambiguous. Is a "3" a request for product number 3, or is the user just being slow at entering "32"? Solving this by adding a leading zero will not work because people are in the habit of ignoring leading zeros.
- Product codes should not coincide with product prices. If there is a bag of cookies that costs 75 cents, users are likely to type "75" when they want the cookies, even though the product code for the cookies is 23.