-william the conqueror can't conquer anything
-if the AI needs vassals help in a war, it actually thinks implementing high crown authority will win them over.
-If you have a bastard child that is legitimized, your son, your son's wife, your son's son, and your wife will all try to kill him. On the other end, your bastard childs mother will try to have her husband killed ( you arranged it to get rid of her), your wife, and your son (not the bastard) killed. Just so she can be with you and her son can be future heir. All of this from one night of sexins.
-Being independent is not always the best option.
-Giving random nobodys land and title cause them to work very hard for you (event wise)
-one last one related to the one up top. If you want to kill a vassal because you will inherit land and title, a good way to get a plot moving quickly is to marry your lover to him. He won't know, and she will want to kill him very very badly.
These are just some that I have learned while playing, never played any CK before so if these seem obvious I'm sorry