Think this one'sa bit of an urban legend but here goes :-

A woman went into the local supermarket and bought two instant barbeques.

The next day, she returned one of these to the customer service desk at the store complaining that there was no food inside as suggested by the picture on the front (the picture showed food cooking on the BBQ).

The assistant told her that these were just the actual barbeques for cooking on and that there was no food inside.

"Oh dear" said the woman. "I'd better take the other one out of the freezer then"!