This is based on one of the ongoing Call of Cthulhu games on niconico douga. In one particular series, Youmu plays a character (Minase Youmu) whose main skills are Horseback Riding, Bicycle Riding and Headbutt.
However, the base damage for Headbutt is 1d4 while due to her low strength and small stature her damage bonus is -1d6. The one time she ever successfully made the attack she ended up inflicting 0 points of damage.
