
So easy to do...
On my current project, I am the responsible for cabling a large system together in an aircraft. I'm a mechanical aerospace engineer, and cables apparently belong to us, but that's another topic.
It is super easy if you don't pay attention to have connections not match correctly, due to incorrect or inconsistent naming conventions. What's a transmit signal on one device will be a receive on the other, what can be a primary signal for one device is only a secondary for another. You really need to have this being controlled at a high System level, with every signal being defined by the System, which allocates it to the lower equipment including to the level of defining exactly which pin on which connector gets the specific signal.
Let the individual boxes in your system control their own naming scheme and you are in for a world of hurt, which sounds a bit like what happened here.
You can't really blame the technicians who connected Main to Main for the issue here, because it sounds correct. Unless of course, it was clearly stated in their documentation, where each connection should be. Actually, there absolutely should have been a wiring diagram they were working to, which would not allow them to connect things up incorrectly without raising questions - which would/should also be checked by an inspector to see that the wiring matches the drawing - so that's a pretty big failure of management to allow any construction without the relevant documentation to make it failure free or the required level of inspection to find the error before it's integrated into the next assembly. Mistakes happen, even with documentation, but if you have the correct documentation, then hopefully those mistakes are picked up before the next stage of assembly...