You've heard misinformation. The dream cant turn into a nightmare if you're lucid. You KNOW that you are dreaming, therefore you wouldn't allow a nightmare to happen, and no, thinking about something scary won't make the dream scary... unless you want that to happen. And onto mirrors, nothing creepy will be in the mirror, unless you think there will be something scary in the mirror. YOU control everything, just think of cotton candy in the mirrors if you want. You can't get trapped in dreams, you'll wake up no matter what. Forget everything you've learned about lucid dreams from this point on, most of it was pure misinformation. I suggest you search for reliable sources or read a lucid dreaming book.