When the Unbreakable tag is applied to an item, that item cannot have a set durability. This means that resource packs that change item textures/models based on durability (a new feature just implemented) will only use the model/texture for the unbroken version when Unbreakable is in effect. This is an issue in the case of mapmaking, because changing models/textures based on durability is used for adding custom items for use in maps.

I'm not sure if this is exactly a bug or intentional (Durability values were never really useful until now), but it does present an issue. If there is a way to make an item stay at a predefined durability level, I would love to know.