Joints do not hold their constraints!! Come on Unity, some thing soooo fundamental.
If you go to the down part of the forums you can enter in the Unity ideas forum
Sadly, this isn’t a unity problem. It’s PhysX. It handles all joints using spring mathematics, sadly for many joints such as hinges, fixedjoint, etc… the spring needs to have infinite force, which means its impossible to resolve perfectly, especially under great stress. There is a system called Articulated joints in PhysX 3.4, which works around this by only allowing objects to simulate within the bounds of their constraint, but unity hasn’t ported to that version yet. Articulations are also slower. =/