Head Butt stability removal gives self weakness if used in fractals with Vengeance instability.
It is very easy to replicate. Just use Berserker and Brave Stride trait that gives stability when entering combat, hit any mob in fractal with Vengeance instability. As soon as your stability pops up (it can be any source of stability at the begining or during combat, Brave Stride is an example for easy replication), hit anything with Head Butt and you will apply weakness to yourself.
Enemy type/tier does not matter, it works even on objects.
In short: If you use Head Butt with stability in fractals with Vengeance instability, you will also apply weakness to yourself.