  We study a gas of $N$ hard disks in a box with semi-periodic boundary
conditions. The unperturbed gas is hyperbolic and ergodic (these facts are
proved for N=2 and expected to be true for all $N\geq 2$). We study various
perturbations by twisting the outgoing velocity at collisions with the walls.
We show that the dynamics tends to collapse to various stable regimes, however
we define the perturbations and however small they are.
