Order of the Phoenix Halloween Party: If Remus looked behind him, he'd know why Hermione was laughing so hard. (And what did Severus have to promise to get Remus to wear that hat?)
Costumes:
Severus: Zombie (with ketchup and spork)
Remus: Pirate (wearing brain hat)
Hermione: Ninja
Harry: Sleeping Beauty
Luna: Saloon Girl
The challenge was "Snape with a spork" - the zombie, pirate, ninja trio is a running joke with a friend.