I had a plot active to remove a duke that didn't support my heir choice. I've already had some of my vassals backing it, when another duke rebelled and this duke joined the rebellion. They lost and as traitors both lost their titles. Now, my plot was stuck as it was ongoing and I couldn't end it, on the other hand, that duke already didn't have the title as it was revoked. So, I had this plot until the king died.