Pig of a crash - "Pure virtual function being called while application was running"

So Im trying to think what I have done that triggers this.

Is it that I am supposed to be overriding a function when I extend a UE class but I have not done?

Or could it be that I am not making a call to the Super:: when I have overriden a class?

I am primarily making subclasses of the basic UE frameworks such as GameInstance, GameModeBase, GameState, PlayerController and PlayerState at the moment