I have seen many posts over the years where people believe that they found some type of a preproduction mockup or rare mispack figure at retail. While the manufacturing plants do make mistakes from time to time, it is good to know how to determine whether a figure has been intentionally repackaged.
Let's start with the most common reason. Many unscrupulous individuals have purchased new figures with the intent to exchange the figure and return for full refund. It's usually because the new figure is either more expensive or more desireable. There is usually a little time that goes into cutting or removing the blister in a fashion to be unsuspecting to the employee handling the return.
Second, there are many customizers out there. They can range from a simple head swap, to a completely custom sculpted and painted figure. Generally custom figures are not usually found at retail. However, they can find their way back to the open market (i.e. Toy Shows, eBay, etc.) Eventually these can be resold as a rare mockup or unproduced figure.
To protect yourself, take a careful look at any potential cuts within the blister itself or possible removal from the cardback. Another point to mention is that generally blisters are heat sealed and NOT glued to the cardbacks.
Here is a video that explains the process and can help you identify any package tampering.