Overhydration.

Yes, it is possible. It's rare, but cases have occured in sport pilots
recently.

The trick is to consume large amounts of water while losing salt. You
can get confused, have seizures, or die (more likely of crashing than
of the water intoxication).

The pilots who are most susceptible to this are those with defective
water/salt excretion: older pilots, those with diabetic or
hypertensive kidney disease, those on diuretics for blood pressure,
especially thiazide diuretics.

Rehydrating with sport drinks should help avoid this. Most people
consume far more salt than they need, which helps make this a rare
condition, but not everyone does this.