Usability tests usually focus on how successful an user is in completing a specific task. While this is often enough for us to determine the success of a design it is not enough to determine if this actually helps the user accomplish their real goal.

Arguably, you can design an user interface that helps the user perform the wrong task in an intuitive and easy way.

Who is responsible for the user's domain knowledge?
The users themselves may or may not know that the task they are performing is the wrong one for their real goals. This lack of knowledge is usually related to a lack of domain comprehension.

This is complicated even further when even in user research sessions users state that they want to complete task X when that task is not the one that will get them closer to their goal.

This means that user experience practitioners need to keep a watchful eye to make sure the user has enough domain knowledge to make quality decisions as they interact with the system.

-Daniel Montano