What a great idea by DDB Johannesburg for Glad. As you may know, Glad is a producer of plastic bags and other products in South Africa that people use to preserve food. The video below explains it in a much more engaging way, but in short, Glad started an instagram account and search for all the food pics that people post. They screencapped them, then photoshopped glad products over the food and sent them back to the original posters two days later, as if the product had preserved the food for that long. They even went so far as to target foodie celebrities as well, gaining some great traction. That is, before instagram became wise to it and shut down the account.
Have a look at the case video below, there's no way you won't feel inspired after watching it!