When a transparent surface penetrates a scattering volume, scatter rays can get stopped at the transparent surface instead of traversing it, regardless of the number of transparent bounces set in the render options.
See the attached file. The 100% transparent plane effectively cuts off volume scattering in the cube. The expected result is that a 100% transparent plane should have no visible influence on volume scattering.
Verified in Blender 2.79, master as of commit 7b29e917118ffdeb39de5c942dd652d40914dbc3 on macOS 10.13 and Windows 10.
I have a potential fix for this coming.