Reproduction steps:
1. Inspect attached GIF's. (Don't mind the green thing in NoError.gif, recording software messing up)
2. Open attached project and scene 0.unity.
3. In the scene view click three times on a "golem mecanim" arm (look at Error.gif).
4. In "Project" window select and expand "golem mecanim" prefab and select "golem mecanimAvatar".
5. In the "Inspector" window click "Configure Avatar".
6. Notice, avatar configuration does not start and there are errors in the "console" window.

Workaround?:(NoError.GIF)
1. In step 3 select "golem mecanim" body only once.
2. Follow steps 4-6, Notice there is no error!

IMPORTANT:
If you delete all the cube objects from the scene before you attempt to reproduce this error, there will be no error regardless of you actions!

Reproduced with: 5.3.0f1 , 5.3.0f2 , 5.3.0f3 , 5.4.0a4.

Did not reproduce with: 5.2.3f1