I'm all for not crossing the streams! But having different connector bodies that prevent cross mating might be an easier solution, and aren't as breakage prone as the pins are. Unless its an older Ford, all you had to do was look at a connector the wrong way and it broke lol!

Pretty damn good guess.
I would bet it was maintenance during heavy check when the panel was off.
A misplaced foot and ..... that's all she wrote.
No blame given Lower pits suck to work in. Getting a good foot/hand hold can be difficult. 

A self holding relay is just a regular engaged/disengaged relay that has been wired to use it's own contacts as a power/ground pathway once it is engaged.

K505 in this image is "holding" its ground path through one set of contacts. As long as the pressure switch is made, K505 will hold itself engaged.
  
  

Different current or signal needs. Different jack/plug sizes are intentionally used. Different keying is used also.

It is not uncommon to have a vertical stanchion that has 50 plugs on it. Most if not all, will be different types and sizes.

Why???? To keep from putting the wrong plug on the wrong jack.

Bad things happen...... like..... crossing the streams.