LTE
Gordy/AC quite right ... or perhaps Lack of Tailrotor EDUCATION ?
Nick is right - there is almost no case of a Tail Rotor which is insufficiently powerful to overcome the torque produced, except perhaps at low RRPM (or EC135 at altitude?). Mostly it is poor pilot technique, and for those pilots it can appear that the tailrotor is inEffective. (However: A tail rotor may easily not have sufficient Authority to be able to Yaw (or sideslip) against an airspeed. Sometimes called Lack of Tailrotor AUTHORITY. but do they need to be flying sideways through the air at a speed that the tailrotor cannot overcome? (rarely) It'll default to straight anyway. Most pilots are caught out at some stage by unexpected yaw, normally by mis-estimating their path throught the air (wind direction))
An example, touching on the subject, here from the thread "ETL backwards": "You could just turn downwind and do a downwind transition but you might not have enough TR control in 30 to 40 knots for that."
L T 'Ed'?