As you are standing there contemplating what to do you feel a sudden tingle and look down to see your fur quickly retreating into your skin and your penis starting to push out of its retracting sheath.
You quickly dart back into the changing room before you are completely exposed. You watch yourself in the mirror as your tail pulls back in and your face turns human.
Soon you are back to yourself again, albeit a little stronger and hairier than you ever were before and no sign you had ever put a costume on.
You pull your original clothes back on and head out the store. As you look up at the store you think to yourself "Duh, its day time and not a full moon, so i didn't stay in werewolf form!" Now you just have to figure out how to force your change at other times...