" You should never have a parent and child rigidbody together " ?

I found this tip in http://unity3d.com/support/documentation/Manual/Physics.html But I would like to know why? What sort of problems can this create?

I don’t think this is valid anymore.
According to official Unity documentation (Unity - Manual: Rigidbody component reference)
When an object is under physics control, it moves semi-independently of the way its transform parents move. If you move any parents, they will pull the Rigidbody child along with them. However, the Rigidbodies will still fall down due to gravity and react to collision events.”

Actually this was only method which properly worked for me when trying interaction of player with platform. No jittering, no issues.

My case:
Parent player with non kinematic rigidbody to platform on collision (OnCollisionEnter),
unparent on CollisionExit.
Setting rigidbody to isKinematic doesn’t help much here as you would need much more sophisticated way to set proper position of player on the platform etc.
You cannot just switch rigidbody to isKinematic on collision otherwise player might easily stay with half of his body under the platform or be affected with a similar undesired effect :wink:
Using rigidbody with physics eliminates this problem as player is either knocked back when jumps to the side of the platform instead of it’s top and is not parented at all or doesn’t miss the platform and is parented immediately.

I haven’t had any problems when parenting non kinematic rigidbody. The movement was pretty smooth.
In all the other cases the player was affected by terrible jittering.

A rigidbody component controls the GameObject it's attached to. The velocity get's calculated in worldspace. If you parent a rigidbody to another, the child will get additional "forces" (actually the gameobject will move along with the parent, so no real forces) that will make the child do crazy things. A rigidbody represents one single physic object. Parent-child relationship doesn't work here. If you want two rigidbodies to be bound together, use joints.