Not at any time. This question relates to a safety precaution for using forklifts. Developed in the early 20th Century, forklifts are powered industrial trucks, which are used to lift and move items between small distances.
They are used in warehouses, construction sites, ship yards, and recycling locations. Forklifts can be fueled by many sources including gasoline, propane, and fuel cell. It is dangerous to stack items on an incline, for the items could fall over, which could cause harm to the products itself or someone else who may be by the forklift.