Unity crashes when there are multiple game objects with colliders and joints being instantiated in the same place. These game objects' colliders must intersect with another collider already in the scene. Game objects must have two Fixed Joint components, one of them being with connectedBody set to existing RigidBody and other one to null.

Reproduction steps:
1. Open attached project
2. Open scene "base" if it's not already opened
3. Enter play mode

Expected: play mode starts
Actual: Unity crashes

Reproduced on: 5.5.0a6, 5.5.2f1, 5.5.2p4, 5.6.0f3, 2017.1.0a2
Not reproduced on: 5.5.0a5, 2017.1.0a3