  The analysis of the time evolution of unstable states which are linear
superposition of other, observable, states can, in principle, be carried out in
two distinct, non-equivalent ways. One of the methods, usually employed for the
neutral kaon system, combines the mixing and instability into one single step
which then results into unconventional properties of the mass-eigenstates. An
alternative method is to remain within the framework of a Lagrangian formalism
and to perform the mixing prior to the instability analysis. Staying close to
the $K^0-\bar{K^0}$ system, we compare both methods pointing out some of their
shortcomings and advantages.
