In our game we build a complex object with a lot of colliders (thus creating a compound collider), then add rigidbody component to it. When it drops to the ground very often it starts bouncing really strange. After a lot of investigation we've found out that it's caused by the center of mass (which is calculated by Unity correctly) and happens even on a single collider. When center of mass is far from rigidbody pivot position, as in our case, it behaves this way. When we move inner objects so that their center of mass is approximately at the same position as parent pivot, the bug goes away. I've created and attached a simple test project to illustrate the issue: open the Main Scene, hit Play, then select the right cube and press Drop on RBGizmos script. It starts bouncing like crazy 80% of the time. Even strangely, when tested in 5.4 beta it just falls through the ground every time.