The Hollow Earth
Carp’s Canny Cap of Confusion
When doffed, the wearer gains the ability to roll two d20s when making a knowledge check and take the higher of the two results.
They also become cursed by the hat, taking a –1 penalty on attack rolls, saving throws, and all non-knowledge skill checks until a Remove Curse spell is cast upon the hat.
The hat cannot be removed by its wearer unless a remove curse spell is cast on it. Once the curse is lifted the hat loses its powers.