Let us imagine there is a container full of small particles which are allowed to collide with each other and the container walls (in 2D). If I initialise the system with given velocities and positions at $t = 0$, and reverse the velocity of every particle at $t = 5$, then at $t = 10$ the particles should exactly return to their initial velocities and positions.
However, when I simulate this experiment with simple 2D physics, the particles fail to return to the initial state if they are allowed to both collide with each other and container walls. So the question is Is my understanding of the physics wrong or is there something wrong with the simulation itself ?
When only collision with container walls or collision with other particles is allowed, the particles more or less return to the same situation. However, when both are allowed they just don't. The only interactions in the simulation are 2D collisions between the particles which conserve momentum and collisions with the "walls" which just reverse the corresponding $x$ or $y$ component of the velocity.