Exceptional bonus

Exceptional bonuses are rare bonuses generally only found on high level loot. Exceptional bonuses stack with all other bonuses, but not with other exceptional bonuses.

Exceptional (Stat) Skills +2, +4 and +6
These bonuses exist for each and every stat and effect all skills that use that stat as their modifier. They only appear on named loot, most notably Green Steel accessories.

Previous exceptional bonuses
In the past, exceptional bonuses stacked in a non-standard way. Between Update 14 and Update 19, all special stacking cases of exceptional bonuses were removed from the game or redesigned:
 * Exceptional Ability +2 => Insightful ability, insight bonus (stacking with exceptional ability) (U14)
 * Exceptional Skills +1/+2/+3 => Effects doubled (+2/+4/+6), no longer stacking (U19)
 * Exceptional Fortification => insight bonus
 * Exceptional Sneak Attack => insight bonus
 * Exceptional elemental resistance +5 => competence bonus
 * Exceptional elemental resistance +10 => insight bonus

The developers have stated that they prefer to offer non stacking bonuses to create more gear options and so do not plan to add more items with exceptional bonuses in the future.