I might have just found an infinite prestige bug.

Still playing as the King of Scotland, I notice that one of my vassals is plotting to kill me. I've never really liked him much (despite him having the name "Murray of Moray", which I personally find hilarious), so I plan to do the standard "arrest, war, revoke". The bug comes in when about halfway through my sieging down his capital, his rival takes advantage of him having his entire army wiped out and launches a rivalry CB war and captures him in battle.

Now, Murray's imprisoned, so I instantly get 100% warscore. However, he's not my prisoner, so he's still free to reject revocations, which he does. Except he's still imprisoned, so I instantly get 100% warscore and enforce demands, netting me 50 prestige. And then I try to revoke again, but he refuses again, and we have another war, which I once again instantly win and claim 50 prestige for. I did this about four times before getting bored and moving on with the game.