The problem with the MCO disaster wasn't mixing up units, it was a sub-contractor failing to follow a specification properly, which can happen for any number of reasons. Also, it's not the only way that such missions can fail. Let us remember that since the mid-90s there have been 2 failed US Mars missions as well as a failed European mission (the Beagle 2 lander), a failed Japanese mission, and 2 failed Russian missions. As well as one and only one successful European mission and 8 successful US missions.