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 and a few crafted items, 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.