You giggle a bit in spite of yourself, thinking that this machine really tries to give you a realistic experience. What was it going to do, take a picture of you and photoshop a tail on it?
Suddenly, something brushes against the back of your pant legs. Wondering if a child went past you, you turn slightly to see what it is. Spotting no one, you turn back to the wish machine, when something twitches at the end of your spine. Maybe your tailbone hurts from sitting too long earlier. You reach behind to rub the seat of your pants, when you notice a blinking message on the machine screen: "Wish granted." At that moment, your hand touches something soft and furry...and you feel the fur on your tail brush against your hand.
"What the - ?!" You gasp, looking down over your shoulder. Sprouting out of the seat of your pants, flawlessly as if it had always been there, was a big, fluffy, orangey tail, all the way down to the white tip. It was proportional in size to your body if you were actually a fox, perhaps even a bit larger. In shock, you stare as it swings back and forth in response to muscles you don't even know how you are moving.
Shock slowly gives way to wonder, and you grin involuntarily. It worked! You have a tail! Wait...you have a tail...how are you going to explain this to anyone? Wonder suddenly fades into a dose of reality. If anyone finds out you are a human with a animal tail, you'll end up as a lab specimen or worse. There has to be a way to fix this.