In my most recent game, I was playing a warmongering sovereign, and when I got to one of the cities, I noticed a Siege button. Unsure of what it did (I'm still not quite sure), I clicked it. A few turns went by, and I noticed that a group of enemies was nearing a poorly-defended city, so I teleported my sovereign and his army near them. However, since I never technically ended the siege (never clicked the siege button to cancel), the game still thinks he's sieging. It won't let him move (the rest of the army can move, just not my sovereign), and, since he's not next to the enemy city, I can't cancel the siege. I captured the city with the rest of my army, but he's still stuck. It's a pretty annoying bug, so I hope it will get fixed.