I've had some reloads. The way I play single-player games, if FUBAR happens, I tend to reload. Anyway:
– Boleslaw II of Poland (starting king in 1066, aged 24), married to Duchess Matilda, four daughters and no sons over 50 years or something;
– Muslims in Spains or rather having to combat them as Poland when they could just re-reraise countless hordes just like that;
– 7k supply limit on the sea when DoWed by the Caliph, plus war everywhere South and East of Hungary (solution: become the freaking master of mediaeval amphibious warfare);
– succession fail (bride of a younger son dies fast, kid is a foreign duke with a liege, guess what happens when younger son, who is a duke here, dies...);
– married a Poland's king's heir to an ugly Duchess of Pomerania to get the land, then de iure Poland (prior to 1.05), without bloodshed; kid became a HRE duke, rebelled, lost it, I now had a foreigner ruler, one of many (I had -18 foreigner for centuries until Queen Blanche the English Toddler got the dialogue to change her culture);
– dumped excessive demesne in Iberia on the heir to end up with queen consort Shakira of Poland, an Andalusian courtier with no family, no stats, no claims, no nothing, who eventually died in prison for attempted murder;
– married Boleslaw II to Duchess Matilda, two sons became HRE dukes on her death, the younger DoWed the elder right away and reduced him to simple courtier (and got himself betrothed to the powerful underaged Duchess of Savoy and some more, guess bastard was en route to forming the Kingdom of Italy but I reloaded);
– as Poland, I noticed that the neighbour of my Iberian possessions was the Golden Horde; I looked up and it turned out that idiot the King of France (and Aragon and Bohemia), separated from them by the HRE (> size 300) and me (size 200 or so) became a vassal of the Golden Horde. WHAT? WHY? H-O-W?
– landed queens consort failing to call husband in a war, losing it (or failing to start a claim war and invite husband into it).