Vending machines are gold for any student of human-computer interaction.
Poor design creates user error, and vending machines are like Norman doors in a number of ways.
When facing a door, the user often has to guess whether to push or pull, and it’s less obvious than you’d think.
When interacting with a vending machine, the user has to guess the correct sequence to use to get the product: is it “pay first, then choose the product” or “choose the product first, then pay”? There is no standard way, and most of the time it’s really hard to guess. Users would typically try “one way” and try “the other way” if they guessed wrong.
The confusion comes from the fact that payments and product selection have two separate input mechanisms, and the user is presented with both at the same time. This would not be an issue if input was done through one input mechanism with a logical sequence, either via existing physical interface or even with a QR code that opens a page that guides users from product selection to payment.
It would be possible to program the machine in a way where starting with either product selection or payment would get the job done, but this is not how these things currently work 
I was recently asked to help complete a purchase with this machine. The customer wanted to buy a bottle of water and thought that the correct sequence is to pay first, then choose the product. However, the customer was confused by “Ice Cold Coca-Cola” message that made them believe that tapping a card would result in Coca-Cola being dispensed. They thought that the “Ice Cold Coca-Cola” refers to a pre-selected product, which they did not want, yet selecting the product that they wanted (water) did not produce any changes on the payment screen. Reluctant to start with payment believing that they would get the wrong product if they did, they were not able to complete the purchase and asked for help.
Ironically, this Coca-Cola branded machine with the “Ice Cold Coca-Cola” message had no Coca-Cola in it at all, but a confusing message, branding and UX almost prevented the customer from completing the purchase.
What would a fix look like in this case? Improving the copy? Reprogramming the input mechanisms or redesigning the process altogether? Adding an explainer? Whatever it is, considering the full life cycle use case is important. There is so much extra context here to be gained from observation that looking at any single step in isolation might lead to wrong conclusions.