Some items have hidden secondary stats. They show up in game as ??? but actually provide stats. So far they are:

Memento: Makes the Lich fight easier (Lich will only have 3/4 of its max health). A message box will also appears at the start of the fight.

Mysterious Mask[1]: DMG +15, INT +10, Luck +5, Dodge +1, Charge Damage +50, Auto Star% +1, Copy spell +4%, Dupe Item +4%

Reckoning Wand: Seems to have a random chance to activate the Star Chunk-effect, which summons a wave of comets with decent damage.

Legend gear appears to have hidden stats, however this third stat is unlocked once the gear is upgraded to level 10.