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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.