A method for preventing either the introduction of air into a liquid aspirated from a container through an aspiration line passing through a container cap, or the over-filling or non-filling of a container by a liquid dispensed through a dispensing line passing through a container cap. Such method comprises the steps of simultaneously detecting that (a) the container cap is properly positioned on and secured to a liquid container from which liquid is to be aspirated, or into which a liquid is to be dispensed, and (b) detecting that the level of liquid in the container is either above or below a predetermined level.