  Quasi-degenerate neutral systems like a (K, \bar{K}) type are investigated in
Quantum Field Theory (QFT). A constant mass matrix as the one used in Quantum
Mechanics (QM) can only be introduced as a linear approximation to QFT. We
study the phenomenological consequences of the differences between the QFT and
QM treatments. The role of ``spurious'' states with zero norm at the poles is
emphasized. The K_L-K_S mass splitting triggers a tiny difference between the
CP violating parameters \epsilon_L and \epsilon_S, without any violation of
TCP. Non-vanishing semi-leptonic asymmetry delta_S - \delta_L does not signal
TCP violation (usual claims not withstanding), while A_{TCP} keeps vanishing
when TCP is good symmetry.
