Multiple Aliances

This is mostly because i tend to go for a peaceful aproach, but when i put some thought to it i remembered the whole imbroglio during our first and second Great Wars and how people with no relations what so ever were fighting or going on sudden embargos because of all the parallel relations going on. The thing is you can have good relations, do trade and have joint projects in diferent fronts, but the AI can always find a way to make things complicated. 

Example:

-You, A, meet the Unfallen, B, early on in the game. You helped each other trading strategic resources and handling those pesky pirates for a good part of the game, A and B start an aliance. And then it happens, you meet a Lumerian, C, that is overflowing with luxury resources. A and B are in good terms and the scientific agreements gave great results, but all those shiny resources are just calling for you. Trade develops between A and C, B even gets a slice of the pie. A goes into a second aliance with C, but then you discover the reason for all those resources. C was playing an agressive expansion style colonizing all over and even invading minor faction systems. Well B is not okay with that, especially after they got pirate marked and even lost a Z'vali system they were courting. they start hostilities and now you have to choose sides. 

You really need your trade with C considering your final development level depends of two traded resources. Also filthy rich and got way too many privateers.

You are too close to B, they have too many fleets close by. And of course you let them entangle your outer systems early on, you were buddies after all, it was a win/win deal. 

Oh, if you stay neutral B and C may get angry and you can lose all.

*this is a very simple off the hat scenario, please be kind.