It's long been known that the most common failure point in a rocket project is the interface between teams. It badly afflicted the early European efforts at a multi-stage rocket because each stage had a different team with a different nationality, and they didn't bother to nail down all the technical details of seperation. Differing units would be a prime example of that, but they could've just as easily goofed by using different coordinate systems (like X,Y,Z=0 being the center of Mars versus the surface landing target).