Kingdom becoming too large or getting a crown I didn't want.
I was playing as an (super)emir in Arabia when I suddenly got the crown of Africa through a faction war. I started imprisoning and banishing people for no reason until I realized I didn't care anymore. So I switched characters to the French duke of Castille. Played for a few generations, and I just got elected King.. not sure I really wanted that and it completely messes up my family domain. Haven't played that save since, started another game as Duke of Deheubarth in the SWMH mod, conquered south wales and southern Ireland, and then I invaded Sweden with the help of Denmark and the finns. Doing well so far, even though I don't really like how the mod translates all the titles to their native languages.