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. 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.
The younger fairies call it 'sorcery sickness' or 'book barfing'.