The U.S. Department of Labor's Occupational Safety and Health Administration
(OSHA) has published a Safety and Health Information Bulletin addressing a
crushing hazard to standup forklift operators associated with "under-ride" that
occurs in a warehouse when an operator backs up too far beneath a storage
rack's horizontal cross bar and protrudes into the operator's compartment.

Forklift truck operators must look in the direction of travel and keep the
forklift under control at all times. One of the potential hazards faced by
standup forklift operators is the crushing hazard that can arise when
traveling, with the forks trailing, in a warehouse near a storage rack or
similar obstruction. The risk is that a horizontal rack beam (crossbar) or
similar obstruction might enter the operators compartment in a situation
referred to as "under-ride." This Safety and Health Information Bulletin (SHIB)
discusses ways to reduce the crushing hazard to the operator associated with
under-ride. Awareness of the precautions and safety measures highlighted in
this SHIB can help prevent serious injuries and fatalities related to warehouse
operations.

More info

AplusA-online.de - Source: U.S. Department of Labor's Occupational Safety and Health Administration