Apparently, many people saw this can and thought it was Diet Coke. The white and silver features of the can the Coca Cola company released as a special edition for the holidays looks somewhat similar to a usual can of Diet Coke. The problem seems to be that instead of paying attention to the text, people tended to act based on the colour features of this new can.
These type of stimulus control errors occur all the time. It happens when we respond according to one shared or similar feature in our environment and not the other features that tell us, “something is different here”. As a behaviourist, I would say the two cans share similar stimuli (red lettering on white background vs. white lettering on red background of a usual can of regular coke) and that the behaviour of reaching for a Diet Coke is controlled by the colour features of the can and not under control of the can’s text. Therefore, people acted as if they were getting Diet Coke because in the past when they grabbed a silver/white can with red lettering it provided them with Diet Coke.
Fortunate for those folks (or not depending on if you view this as an opportunity for people to have learned a valuable skill known as “Always read the labels”), they can go on as the way things were done before because Coca Cola announced they’re pulling the white cans off the shelves.