An "employee" is helping a customer who has a few questions. Not only does the fake employee know absolutely nothing about what is being asked, but the "employee" then "accidentally" breaks an item in the store while trying to demonstrate to the customer. Then a "manager" walks by, and the employee blames the broken item on the customer. The results are endless. People might react in all sorts of entertaining ways. The prank really depends on the fake employee's ability to keep cool while lying to the manager, as if he does this all the time. I think the prank has the potential for different outcomes based on how the manager reacts to the "situation". I also thought it would be funny to let the "manager" deal with the customer who "broke" the item, and let the salesman go on to talk to other customers, and then seem to be a knowledgeable, decent employee.
I believe this could be a very fun prank that could be done a number of different ways, and would provide for hilarious reactions from confused and irate customers.