Two members of LEPRetrieval succumb to the effects of not having permission to enter Fowl Manor.
The Rule of Dwelling, established by King Frond in The Book of the People, forbade fairies to enter human buildings without an invitation. If a fairy breaks the rule, he or she will get sick, with symptoms including nausea and vomiting. If one does this repeatedly, he or she will lose his or her Magic. The loss of magic does not effect the gift of tounges, as Mulch Diggums uses the gift of tounges multiple times despite having broken the rule. In The Time Paradox, N°1 unravels the spell that caused it, as the Rule of Dwelling posed an obstacle to LEP officers. The new rule is introduced sometime in between The Lost Colony and The Time Paradox.
In The Eternity Code, Artemis Fowl II and Holly Short used the rule to their advantage.
The younger fairies call it 'sorcery sickness' or 'book barfing'.