If a secret police agency from some nation's government was ever to read the CK2 forum and especially the thread titles, they would get goosebumps and shiver from fear right then and there.
Regardless, I once played an 18 year old king of France who had a child with his courtier's wife not long after her marriage. That child was not of his dynasty for some reason (probably the father considering that child as his own? I am unsure...). Around 19 years later, that bastard girl who was actually the king's own daughter grew up and became his lover. Then he seems to have another girl, at which point I became confused as to what should I call this one. Granddaughter? Daughter? She turned out to be a bastard as well, but still unsure if it was his own child or not (her mother was unmarried).
The lover died after a few years, and the king spent a lot of years alone. But then that granddaughter/daughter too came of age, and within five years she was his lover. He gave her a successful tumble and croaked after a month. My new ruler, that hedonist king's real legitimate son, soon noticed that a new bastard appeared in the court near the end of the year, actually being his father's son and great-grandson. At that point I was awake for two days without break, and it became so badly confusing that I voluntarily quitted the game.
Argh, if only I had taken screenshots...these relationships are ever more confusing and complex than Julio-Claudian dynasty's adoptions.