  The one-dimensional dynamics of a classical ideal 'exotic' fluid with
equation of state $p=p(\epsilon) < 0$ violating the weak energy condition is
discussed. Under certain assumptions it is shown that the well-known
Hwa-Bjorken exact solution of one-dimensional relativistic hydrodynamics is
confined within the future/past light cone. It is also demonstrated that the
total energy of such a solution is equal to zero and that there are regions
within the light cone with negative $(-)$ and positive $(+)$ total energies.
For certain equations of state there is a continuous energy transfer from the
$(-)$-regions to the $(+)$-regions resulting in indefinite growth of energy in
the $(+)$ regions with time, which may be interpreted as action of a specific
'Perpetuum Mobile' (Perpetuum Motion). It is speculated that if it is possible
to construct a three-dimensional non-stationary flow of an exotic fluid having
a finite negative value of energy such a situation would also occur. Such a
flow may continuously transfer positive energy to gravitational waves,
resulting in a runaway. It is conjectured that theories plagued by such
solutions should be discarded as inherently unstable.
