Players who are Superallergic, when they swallow a Peanut, will suffer a terrible Allergic Reaction, affecting their stats in a heavily negative way.
For the rest of the Discipline Era and most of the Expansion Era, Superallergic never activated. The Players avoided swallowing stray Peanuts, and could eventually lose the Superallergic modification via the Ballpark Renovation Peanut Mister.
But during the Season β18 Playoffs, the first instance of Superallergic happened. Nerd Pacheco, pitcher for the Philly Pies who was Superallergic at the time, swallowed a stray peanut on Season β18 Day 107. This resulted in a Superallergic reaction, which in turn resulted in a combined 15.9 → 0.5 stat decrease.