All 33 of the Ground-Based Midcourse Defense (GMD) interceptors had a serious defect that could render it useless in combat, the U.S. Government Accountability Office (GAO) has warned.
By USAF [Public domain], via Wikimedia Commons
Furthermore, ten of those interceptors and eight waiting for delivery had another defect as well.
The most serious flaw is with the divert thruster, designed to maneuver the kill vehicle to hit its target in space, the GAO found that the original thruster did not live up to expectations. Plans to switch to an alternative thruster had hit a roadblock. All current and future interceptors had to rely on the current thruster while waiting for a new thruster to come online.
A second serious problem is with the wiring harnesses that powers the kill vehicle. A subcontractor had used unsuitable soldering material to assemble the harnesses. Over time, the harnesses could fail during flight.