The title pretty much tells it all.
I noticed couldn't add the trophy onto one of my weapons... At first, I thought it was that it was because I had enchantments or because I already had 3 modifiers. But I did a lot of testing and I've narrowed the issue to this: The Headhunter's Trophy can't be placed on weapons with a bane modifier.
Maybe this is intentional, but I doubt it considering the Flame of Udun and Chill of Daidelos *can* be used on weapons with bane modifiers.
That's all, thanks,
-Phoenix