[Gladsomebeast]

Each frame, the physics engine updates the position of the balls, then checks if the balls are touching other solid stuff. If the balls move so fast that they are on the other side of a wall, free and clear, then no collision is detected.

A thicker wall makes is more likely the ball is within the wall, a collision is detected, and the balls stay in the box.

Should fix things, but the physics system can be kind of crazy.