Human error blamed for Vega launch failure

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Joined Aug 27, 2009
However, “straightaway after ignition” of the upper stage, he said, the vehicle started to tumble out of control. “This loss of control was permanent, inducing significant tumbling behavior, and then the trajectory started to deviate rapidly from the nominal one, leading to the loss of the mission.”

Analysis of the telemetry from the mission, along with data from the production of the vehicle, led them to conclude that cables to two thrust vector control actuators were inverted. Commands intended to go to one actuator went instead to the other, triggering the loss of control.

“This was clearly a production and quality issue, a series of human errors, and not a design one,” Lagier said.
If you can accidentally swap the cables and have them still fit into the wrong connectors, that IS a design error for something as important as critical thrust vector control. Different termination connectors, harness asymmetry (won't reach the incorrect component) in line/cable length, blocking pins and sockets in connectors, bright color matching codes for terminations are some of the things commonly used to prevent assembly errors.