Would that mesh happen to be a terrain?
Terrains change their meshes dynamically as you move around. A polygon which had a sharp indentation a moment ago beneath the sphere, may suddenly get smoothed over as more detail is added, resulting in the sphere interpenetrating the mesh. Interpenetration breaks the collision system.
Of course I'm just guessing that Mark didn't code some workaround to this issue into his terrain system.
Terrains change their meshes dynamically as you move around. A polygon which had a sharp indentation a moment ago beneath the sphere, may suddenly get smoothed over as more detail is added, resulting in the sphere interpenetrating the mesh. Interpenetration breaks the collision system.
Of course I'm just guessing that Mark didn't code some workaround to this issue into his terrain system.