Employee was on top of a hot oil tanker trailer attempting to remove the lid to fill the tanker. The lid was stuck due to sitting for a couple of weeks because of inclement weather. The fall protection cage provided at the fill location did not fit the tanker trailer properly, leaving a gap large enough for the employee to fall beneath the cage. When the lid came loose, the employee fell from the top of the tanker trailer, under the fall protection cage, to the ground. The employee dislocated a shoulder, broke two to three ribs, and received contusions.