Aaagh, stop causing long email threads because you:
1) Don't understand each other's business processes and can't tell that the no big deal little exception you made is going to cause an issue in the other department
2) Don't explain clearly the issue the first department caused for yours
As much as I hate to see the thread, I'm glad when I end up in the middle and can explain to them why they don't understand each other.
It makes me feel useful when my professional curiosity and all the asking and listening I've been doing for years pays off. π