Big Varus W Bug with Red Buff - Passive procs automatically

If Varus kills a Red Buff with W stacks on it with Q or E, and Q or E's damage alone is enough to kill Red, Red Buff's ticks will proc Varus's W passive stacks without needing further Q/Es.

Reproduce steps in the video.

All credit to Vandiril for finding this bug and showing how to reproduce it.