From Physics Simulation Wiki
In a discrete dynamics world, (i.e. when not using Continuous Collision Detection), objects are moved to a new place each tick, and then collision tests are performed. This means that the object, if fast enough, can move from one side of a wall to another without any collision being registered. This is called tunnelling and is usually a bug. It can be fixed by using Continuous Collision Detection or by Stepping the World at a faster internal rate.