Problem with EU4 is that it's unable to surprise you. They are rigged games to favor certain trends and progressions so any WTF moment is likely a bug. Since I think the topic isn't about your own achievements and more about whatever the system manages to spit out by themselves then here are thee stand-outs.
Aberrant decision.
Scotland having Osel (Livonian Order)
Austria having bulgarian regions/norway from a peace deal without a costal province themselves.
Riga became the Papal State
Then there are the internal tag identification mix-up.
Cored an african costal province as Hosakawa and when completed it was given to Spain. I don't mind the early westernization but you just gave me all the bad wester influences that destroys japanese empires.
Everything else is hard-coded bullshit to keep that experience on historical rails.
Natives, western influences in Japan, events, biased modifiers etc, etc.
Other bugs: My ruler had an progenial heir at the age of 8.
CK2
I don't play dating sims. It's more random and thus more interesting but the gameplay is just too centered on finding everyone a good lay that I get bored. Maybe I just haven't found a suitable playstyle, it certainly plays favourites like EU4 does.
HoI - you thought EU4 was rigged?
I've played one real game of the game. As Lithuania I: got into the axis powers, allied japan, got instagibbed by russia, the mrp did not apply to Japan so they went to war early, proceeded to tear through costal russia into siberia, meanwhile the real war with the reich and russia started, the occupation changed, the western front ground to a halt at the designated -historical- location, japan reched over the urals and started to work toward the middle-east. Japan peaced out, taking a dissapointing chunk of russia - coast and some mainland, germany started to give ground, I got bored and spent the rest of the evening doing chores.
Ah, since this isn't anything about personal achievement - my sole contribution was getting Japan into the war against russia when they decided to surprise ******** the baltics. So the most interesting thing in the game was seeing Japanese resistance fighters in Lithuania. womp womp