pole zero cancellation

say a controllable system uses state feedback to cancel a zero in the lhs of s-plane. the system becomes unobservable. but does this cause ne probs. i think not. since at the start the system was controllable and observable(my assumption) what harm does it cause if now the system becomes unobservable. u need an observer only if u can't measure one or more state; so if u start with an observable and controllable system there is no problem in setting up a state feedback controller using an observer.