When I worked for a glass company, we played a joke on the business next to us. We had to replace a bottom glass panel on their storefront. Had to take the large panel above off to do the work. When we put the top panel back up, we flipped it so there window signs were facing in. It took them 2 days before they realized it was flipped. The owner said he never came in the front and from the back never realized what had happened. Would have been longer if it wasnt for a customer pointing it out to him. This guy and us played a lot of practical jokes between the shops.