An employee was retrieving a pallet from the upper shelving of the pallet rack (approximately 40 feet) using a pallet jack. As he raised the pallet, another pallet on the backside of the rack was overlapping and began to tip. As the second pallet tipped forward four wheel rims and the wooden pallet fell from the rack, one of the rims struck the employee's head. The employee sustained bleeding and swelling of the brain.