Dealing with an Ill-conditioned Matrix Error

What causes the warning message? 

This warning message means the stiffness matrix of the model is ill-conditioned. Some elements have severe differences in stiffness from other elements.

The ill-conditioned matrix can occur due to multiple reasons, such as rigid body motion, highly distorted elements, high frictional contacts, units that may result in numbers too big or small, or unstable material models.