So, I'm at war (a large war with multiple allies on each side) against my own vassal. This is bullshit.
I declare war of reconquest against player, multiple other players become involved, multiple territories start getting exchanged in course of war. All is well. Then one of my vassals who has a link to the player that I declared reconquest on has HoH died and I gain a single Duke title. That title makes me liege of the person I originally declared reconquest on.
War continues, all players still involved and territories still exchanged. In fact I have over half dozen territories that I can't reclaim now tonight as planned because I am liege to the holder, but we still at war. Also means my allies are vulnerable against him and his allies.
Someone obviously didn't think this through when making the game.