2D collision filtering is broken.
Regression - not present in 4.3.0b2 and 4.3.0b1
Reproduced only on 4.3.0b3
2) How can we reproduce it using the example you attached.
For Win 7:
1. Open the scene.
2. Press play. Notice the cubes do not collide.
3. Open Physics2DSettings(in menu Edit->Project settings) and look at the Layer Collision Matrix. You can see that Player/Ground collision is checked, so they're supposed to be colliding.
4. Check the box for Player/Player collision.
5. Press play. Now they will collide. This makes no sense.