There is so much conflicting information and old wives tales around this subject it is hard to know what is true. There at a few things, thought, that seem common to almost all you read.
- Don't accept a gift from the fey.
- Don't consume the food or drink of the Feywild.
- Never dance with a fairy.
- Never tell a fairy your full name.
- Never stray from a path in the Feywild.
- Never give a fairy clothing
- Don't forget your manners, but never say "thank you"
- Always keep your word.
- Don't repay more than you borrowed.
There are negative consequences for disobeying these rules around fairies but again, what might happen does not seem to be somethign any of the books can agree on. Such things as "time moves very quickly the more you break the rules", "you become a fairy's slave", "become hoplessly lost in their land" right up to "One you leave, you can never again feel happy untill you return"