Either Identify the normal force incorrectly or mistake the frictional force as an active rather than an reactive force.
where mu = coefficient of friction, either static or kinetic.
For example, when the object is being pull by a vertical force
which is less than the weight.
Another confusion is the switching from state friction to kinetic friction. If the force applied not large enough to overcome the static friction, the object should not move. Instead, some innocently assume that the frictional force overpowers the applied force, rendering the object to travel in the direction opposite to the applied force, mistaking the friction as an active force rather than an reactive force. A reactive force can not be greater than the force applied to the object.