"Hoo, boy," your uncle sighs. "That's a good question. I really couldn't tell you right now."
"You mean...you don't know how to change me back?" you gasp. "But...why did you have me put on a costume?" You're perturbed by your new feminine voice, but you're more worried about the fact that your uncle, a wizard, doesn't know how to change you back.
"Well," he replies, "I really wasn't expecting you to put on a girl's costume. They're designed to change the wearer to match the costume, but I didn't design them for trans-gender transformation, and I hadn't even got around to testing that yet. So obviously something is a bit haywire with this one. It might be something as simple as having over-powered the transformation spell; if that were the case, it'd be quite easy to change you back."
You smile, hopeful, but his expression isn't very reassuring. "On the other hand," he says, "it might be something complicated. And if I go barging in on a complex bug with a simplistic, brute-force fix, God knows what would happen. You might be changed further; maybe you'd be ten years old, maybe eighteen and pregnant, maybe you'd become some monstrosity from beyond space and time; who knows? Maybe reality would change so that you'd always been this way. Maybe you'd change along with it and forget you ever were male. Most likely, the change would just be made absolutely irreversible. There's absolutely no way to tell."
You gasp, trembling in terror at the thought of these fates. "Then..." you say, "...what can we do?"
Uncle Jack sighs. "I can work on probing at the remains of the costume's magic and trying to figure out what went wrong and how to fix it," he says. "You, I'm afraid, are just going to have to live with it until I do."
"H-how long will that take?" you ask, not sure you really want to know. Your uncle shrugs. "Days, weeks, months. Who knows?" he says. "Debugging magic isn't exactly a quick-and-easy process. I just hope I can get this figured out before your parents get back; I can't even begin to imagine how I'd explain this to your mother."
You can't believe what you're hearing. Weeks or months, stuck as a fairy? A girl fairy? How could you possibly be able to "live with it?" You look over yourself as you flutter in the air. "Even my wings are girly," you moan.
"Those you can change, at least," Jack says. "Just concentrate on them." You do, and to your surprise, they quickly change color and shape, becoming blue-tinted crystalline dragonfly wings; if nothing else, at least those are a little more gender-neutral.
"All right," uncle Jack says. "Make yourself at home. I'm going to get a start on figuring this out, but I'll need whatever you can remove of the costume."
You're not exactly thrilled about hanging around in the buff, but it's much more important to get this fixed as soon as possible, so you disrobe. "There might be a box or two of your mom's old doll clothes around here somewhere," your uncle says. "Otherwise, I'll work up some clothes for you when I get a free moment." With that, he goes to his workshop, leaving you to face life as a fairy...indefinitely.