Last week's crash of an unmanned Proton rocket carrying three GPS-like satellites is still in the early stages of investigation, but a possible cause as reportedly been identified, and it's a doozy. Apparently some of the flight navigation sensors designed to measure the rocket's angular velocity were installed upside down. This would have reversed the direction of all their measurements and fatally disoriented the flight computers. The kicker? Each sensor had an arrow printed on it, designed to point towards the top of the rocket. When crash investigators arrived on the scene, they reportedly could still see the arrows pointing down in the wreckage.
This is the sort of thing that simply must be caught in the quality control process. Rockets and spacecraft are extraordinarily complicated machines, assembled by hundreds or thousands of people. Quality control issues were blamed for the 2011 failure of Russia's Phobos-Grunt mission to leave Earth orbit and, if errors like this aren't being caught, it's not terribly hard to imagine why. With Russia beginning a new period of intense investment in space technologies, they will need to overcome systemic problems like this to really make progress.
Source: Russian Space Web