There can be some rather... odd AI interactions within the game. For example, as an Irish King, it's possible to (through bloodline ties to the throne) become nominated by a Faction to become King of England... and actually attain the throne. Granted, it's likely that your dynasty will be kicked off as soon as there's a Succession Crisis, but for that particular King's lifespan, you'll be wondering how the hell it happened.
It is not only limited to courtiers. Female rulers can seduce other male rulers too. Your husband will get a -200 opinion penalty because you are a harlot and will confront you while throwing furniture around. The female ruler can then tearfully reassure him her love and proceed to seduce other guys.
After Sunset Invasion some forumgoerers grumbled that maybe the next DLC should have undead vikings if they're going totally ahistorical. Then in the Old Gods DLC, the message for obtaining the "Viking" trait mentions rumors of <charname> and their armies of undead raiders.