3. You must have a valid park ticket
This seems like a given, but it appears that people have found a loophole in the system to be able to make FastPass+ selections using tickets that aren’t valid. (Don’t try to do it, though, because Disney has already said they’re cracking down on it.) You should have a valid park ticket when making your FastPass+ selections and keep it on hand when you go to the attractions you chose as they will ask you to scan it to make sure you have a fastpass.
