I'm currently playing a game where I have grown quite big, however one of the AIs has a single province on the other end of my territory. Since they got it, they have very frequently sent 4 unit parties into my land, where they seem to get stuck after a short amount of time. They move inwards for 2 turns, and then spend the rest of the time moving back and forth, slowly but surely killing themselves through trespassing attrition. Usually they end up going down until there is 1 unit left, before they fix themselves and move back into their own land to heal up, recruit new units, and repeat the process.

Should've clarified, their main land is on my southern border, however they have a single province to my northeastern border.