The aft pressure bulkhead is a component of all large commercial aircraft. It is an airtight bulkhead located between the cabin and the tail of the aircraft. Its purpose is to seal the rear of the plane and thus maintain cabin pressure, and as such it is a vital part of the aircraft.
Japan Airlines Flight 123 crashed after a catastrophic failure of the aft pressure bulkhead. The failure occurred due to faulty repair of the bulkhead, when a double row of rivets was replaced by a much weaker single row. Failure of the bulkhead damaged hydraulic pipes passing through.