I don't have a save file handy since it was a while ago. All you would have to do to reproduce the bug is have two different civ's cities close to each other and be at war with one or the other. Using a thermonuclear device (or a regular nuke if close enough) on the civ you're at war with on a tile where the second civ's borders are within the blast radius will cause a pop-up telling you this would declare war on the civ you're already at war with. If you proceed and attack anyway the civ whose borders you just contaminated will now be at war with you as expected. I assume the problem is the variable name in the Declaration of war pop-up is that of the target tile owner. Note, I haven't yet but am curious to see what would happen were I to target a neutral tile between two or three territories.