  After general considerations about limits of theories and models, where small
changes may imply large effects, we discuss three cases in galactic
astrophysics illustrating how galactic dynamics models may become insufficient
when previously neglected effects are taken into account: 1) Like in 3D
hydrodynamics, the non-linearity of the Poisson-Boltzmann system may imply
dissipation through the growth of discontinuous solutions. 2) The relationship
between the microscopic exponential sensitivity of N-body systems and the
stability of mean field galaxy models. 3) The role of quantum physics in the
dynamics of structure formation, considering that cosmological neutrinos are
massive and semi-degenerate fermions.
