Interesting bug:
1. Conquered city from Korea.
2. It was a holy city, so it revolted -- I granted it independence, it became Vietnam.
3. At some point, perhaps because of the independence, Vietnam had an open borders agreement with me.
4. Vietnam eventually became a vassal of Khmer.
5. Even though I could get rid of open borders agreement with Khmer, I could not with Vietnam -- there was no option to cancel it in the "current agreements" part of diplomacy.
6. Eventually I went to war with Khmer and Vietnam, and even though our units were fighting, we still had an open borders agreement, which led to some interesting results, including units not being removed from each others' territories after DOW.
7. Finally fixed problem by eliminating Vietnam.
Weird.