In my sub Behaviour Tree
during the transition between animations
(specifically Stand_to_Crouch_Rifle_Ironsights and Crouch_Idle_Rifle_Ironsights)
the character pops to his default standing stance for one frame.
I determined this definitively by using Pause and then Frame Skip.
It happens in the video too @3:11 @4:25 @4:40 but Ben mentioned it will be address in a future lecture.