An employee was loading a pneumatic roller onto a flat bed trailer. Once the roller was centered, the employee attempted to place the roller in park, but hit the wrong lever. The roller started to move in reverse, so the employee tried to put it back into forward. The rear wheels started to spin and the roller fell off the flat bed trailer. The employee held on as the roller was turning over and hit a mailbox. The impact with the mailbox broke the employee's left femur.