Boeing said that the damage came from "incorrect shimming performed on support structure on the aft fuselage of some 787s. Shims are inserted by mechanics to close small gaps between parts that do not fit together exactly. The delamination is taking place in a section of the aft fuselage near where the horizontal tail is joined.

 

A similar faulty shimming problem had taken place in 2010 when mechanics working for Alenia were building the horizontal tails of 787s in Italy. While tightening the fasteners, they had applied too much torque resulting in degradation of the composite material due to the compression of the shims. The damage had taken place at the horizontal tails resulting in months of delays to the jet program. About 50 Dreamliners have been assembled completely to date and five have already been delivered by Boeing to All Nippon Airways of Japan.