Residual Entropy - Third Law I've been told that many systems possess some residual entropy at absolute zero.
This would seem to disagree with the 3rd Law of Thermodynamics? How can this be explained physically speaking? 
I am mildly aware that the 3rd law speaks with respect to a perfect system but I've never really understood what is meant by this.
 A: I think it's not purely an issue of degeneracy, from what I understand a system can have a residual entropy even with a unique ground state. I'm not sure, but I think the way it works is that every system would end up in its lowest-energy ground state at exactly absolute zero, but in practice if we are interested in the behavior as you approach absolute zero, some systems may have a large number of possible states close to the ground state so that if you lower the temperature and measure energy as a function of temperature, the system may not have enough time to "find" the ground state and spend more time in it than other states as T approaches 0. In this case, if you lowered the temperature over some sufficiently huge span of time it would still be true that the average energy as a function of T would approach the ground state energy as T approached 0, but for a more practical span of time in which you lower the temperature and measure how the energy changes (by measuring the heat capacity, which is the rate of change of internal energy with respect to temperature), the energy may not approach the ground state energy on a graph of U vs. T.
Here's a discussion of residual entropy from An Introduction to Thermal Physics by Daniel Schroeder, p. 94 (note the part I bolded below about needing to 'wait eons' for the system to find the ground state, and the other bolded suggesting there is a unique lowest-energy ground state for the system he's discussing):

In practice, however, there can be several reasons why S(0) is
  effectively nonzero. Most importantly, in some solid crystals it is
  possible to change the orientations of the molecules with very little
  energy. Water molecules, for example, can orient themselves in several
  possible ways within an ice crystal. Technically, one particular
  arrangement will always have a lower energy than any other, but in
  practice the arrangements are often random or nearly random, and you
  would have to wait eons for the crystal to rearrange itself into the
  true ground state. We then say that the solid has a frozen-in residual
  entropy, equal to k times the logarithm of the number of possible
  arrangements. 
Another form of residual entropy comes from the mixing of different
  nuclear isotopes of an element. Most elements have more than one
  stable isotope, but in natural systems these isotopes are mixed
  together randomly, with an associated entropy of mixing. Again, at
  T=0 there should be a unique lowest-energy state in which the
  isotopes are unmixed or are distributed in some orderly way, but in
  practice the atoms are always stuck at their random sites in the
  crystal lattice.
A third type of "residual" entropy comes from the multiplicity of
  alignments of nuclear spins. At T=0 this entropy does disappear as
  the spins align parallel or antiparallel to their nighbors. But this
  generally doesn't happen until the temperature is less than a tiny
  fraction of 1 K, far below the range of routine heat capacity
  measurements.

A: In the ideal case, at zero kelvin the system must be in a state with the minimum possible energy, and this statement of the third law holds true if the perfect system has only one minimum energy state, called the ground state. Entropy is related to the number of possible microstates. If the ground state of the system is degenerate  system containing a certain collection of particles, it is possible that the grounf state is degenerate (tat is, more than one ground state exists). In such a case the entropy could not be zero, aven at zero K because there will be more than one microstate compatible with the ground macrostate. 
