Non-adherence to ATC procedures
Procedures in an average Air Traffic Control manual constitute a rather complex set with several exceptions and specials. Controllers do not always apply them, at least not to the letter.
Indeed, why not use a bit of the safety margins in the separation minima in a very controlled way to increase efficiency? Sometimes the strict formulation of a rule does not suit the actual, dynamic operation, as several interests are at stake.
A white paper "Non-adherence to ATC procedures, The most fruitful input to safety management", by Bart Klein Obbink can be downloaded here.
Non-adherence to procedures has its dangers, however. The organization might drift, passing the boundary defined by the procedures, without knowing it till incidents occur. The organization might therefore slowly move towards the invisible, real safety boundary, beyond the control of the safety management system.
It might therefore be wise to verify ATC procedures themselves. Are they clear, consistent, necessary, comprehensible and precisely formulated? Are some deviations acceptable, for example as they are small or related to higher priorities?
More difficult, and more sensitive, is to investigate how far ATC procedures on paper are from the application in practice. In a last step, the adherence to procedures is continuously monitored as part of the Safety Management System. That is not easy for several reasons, but might be much more beneficial than only counting incidents.
A white paper on this topic can be downloaded here. The paper addresses the relation to safety management and how the risk related to non-adherence can be managed, e.g., by monitoring appropriate safety performance indicators.