To reproduce:

1. Download the asset RPG Monster Animation (https://www.assetstore.unity3d.com/en/#!/content/79385)
2. Select skeleton_archer@attack.fbx asset
3. In the Inspector, in Animation tab, change something and press Apply

Expected: applying changes to .FBX file doesn't show any errors.

Reproduced in 2017.1.0b10, 2017.1.1f1, 2017.2.0b10
Did not reproduce in 5.6.3p2, 2017.1.0b9, 2017.3.0a1, 2017.3.0a7

Not reproducible since 2017.3.0a1