Some post-patch buggy diplomatic interaction:
I've spied on William before, got caught and promised not to do it again. I had the dark spying diplo hit. I've moved my spy elsewhere.
Then William gets caught spying on me and his spy gets killed by mine.
Instead of the usual apology screen and the usual choices William denounces me and I get the modifier for having broken my promise not to spy again.
The save is from a few turns before this happens.
I've spied on William before, got caught and promised not to do it again. I had the dark spying diplo hit. I've moved my spy elsewhere.
Then William gets caught spying on me and his spy gets killed by mine.
Instead of the usual apology screen and the usual choices William denounces me and I get the modifier for having broken my promise not to spy again.
The save is from a few turns before this happens.