Operating system: Kubuntu 18.10
Graphics card: GTX970
Broken: 2.80, 3600bb79d05d, 2019-03-15
Short description of error
In a scene with an animated camera which is tracking to an animated target, motion blur 'explodes' on certain frames.
Exact steps for others to reproduce the error
I have a .blend file attached; it contains a simple scene with a cube, an animated camera tracking to an animated empty. When rendering frame 39 with Cycles and motion blur enabled (default settings), the motion blur 'explodes'. Other frames seem fine. If I disconnect the Environment Texture node from the Background in the world shader, the motion blur seems fine.
I am not 100% sure this isn't some sub-frame gimbal lock issue I should avoid in animation, but since it manifested often with Blender 2.8 I suspect this is a bug.