Consider this. In season two of Buffy the Vampire Slayer, the villain Ethan Rayne turned every one who wore his Halloween costumes into their Halloween costumes. Willow, who was dressed as a ghost literally became a ghost. Buffy who was dressed as an 18th century upper class lady became such a woman. Our usual hero the slayer was helpless, confused and pretty much useless when all those kids dressed as demons, werewolves and goblins actually turned into demons, werewolves and goblins.
Thankfully one Mr. Xander Harris dressed up that Halloween as a soldier. When the spell took effect Xander was transformed into an actual soldier with a plethora of soldierly knowledge, skill and courage. It was quite fortuitious. He saved the day often on fateful that Halloween night.
So this is definitely something to seriously consider when shopping for a Halloween costume. What if a disgruntled former friend of your watcher who worships chaos shows up and casts a magical spell that turns your entire town into whatever costume they're wearing. When that happens what will you become?
You don't want to get stuck as a schnauzer or a ladybug.