If the Additive layer in the descriptor is overridden but no animator is put in the slot, gesture based animations become randomly broken. Each time upon loading the avatar, there is a roughly ~50% chance that gesture parameter state changes will be fired with ANY state change happening in the animator, regardless of animator layer or parameter driving the transition. This chance also happens for remote clients, and is local to each client. So one person may see it broken, while another may see it normally.
Here's a video showing the bug in action with debug menu, and how it has a random chance of happening upon loading the avatar. In the video, the animation that is being effected is simply fired when GestureLeft == 1 and resets when GestureLeft =/= 1.