A heavy ladder resting on floor and against a vertical wall may not be in equilibrium, if
When a ladder rests on a smooth horizontal surface and leans against a rough vertical wall.
As seen from the diagram, the lower end of the ladder will start moving horizontally due to the gravitational pull of the earth towards the ladder.
A heavy ladder resting on the floor and against a vertical wall may not be in equilibrium if the floor and wall both are smooth surfaces.
As the surface is smooth, there would be no frictional force on the smooth horizontal surface.
But as the ladder moves horizontally, the upper end of the ladder starts moving down and this motion of the ladder down will be opposed by a frictional force in the upward direction as the wall is rough.