What is error in action? And why is it called error "in action".
From Manual of Gear Design: Spur and Internal gears, volume II:
The load carrying ability of a pair of gears may be limited by either the beam strength of the gear teeth, or the surface endurance limit of the material. The lower of these two values will be used to establish the load carrying ability of any pair, and must be greater than the maximum dynamic load.
Dynamic Load - whenever there is an error in action between a pair of gears, one of two reactions must take place: the connected masses must be accelerated and decelerated to compensate for this error, or the system must be elastically deformed the amount of the error. In practice a combination of the two will occur. Even with perfect gears, there will be an error in action due to the deformations under load. The load caused by these reactions is called the dynamic load.
It's called "in action" because the error is not present when the gears are not under load.