A "nightmare scenario" is a situation where everything goes wrong beyond all expectations. Legends of magick are rife with such stories. Perhaps the most famous occurs in Goethe's poem, Der Zauberlehrling, which was popularized in Dukas' symphonic poem, The Sorcerer's Apprentice.
In the story, an apprentice magician casts a spell on a broomstick to fetch water. When he doesn't know how to stop it, he chops the broom into pieces. Unfortunately, each piece turns into a new broomstick and continues to bring a flood of water. Only the return of the apprentice's master, who breaks the spell, saves the day.
If you're doing some form of practical magick, how can you avoid a nightmare scenario? Some people include the concept that the magick should be for the greatest good and harm none. I don't like this because the "greatest good" might be the failure of the magick. I contend it's better to find out what the results of the magick will be so you can choose to go ahead, modify the ritual, or abandon it.
In my opinion, the best way to find out how to discover the outcome of magick would be to perform a divination; one way to do this is by using the Tarot. The Hermetic Order of the Golden Dawn had five basic types of magick, and they thought that working with the Tarot was so important that they dedicated one type of magick exclusively to Tarot divination.
When performing a divination of this type, don't simply ask, "Should I do this ritual?" That disempowers you and lets your decision-making be done by pieces of paper with pictures on them. Instead ask, "What will be the result of doing this ritual?" Based on that information you can decide whether you should perform the ritual as designed, alter it, or do something else.
Working with the Tarot isn't difficult, but it takes a bit of training and practice. If you're just starting, I'd like to suggest the Easy Tarot boxed set. There are a couple of reasons for this. First, the book is very clear and focuses on the practical aspects of the interpreting the cards rather than on history, theory, or philosophy. Second, included in the box is a breathtaking deck, The Gilded Tarot. The art is fantastic and colors are brilliant. Although it is consistent with traditional symbolism it is also so unique that you'll know you are doing something special.
If you're more advanced, you'll want to know more about the cards, their relationships, and how to design your own layouts. A perfect book to develop these skills is Tarot: Get the Whole Story. It's like watching a Tarot master at work, and the sample readings are a lot of fun.
Finally, for advanced Tarot readers, I suggest The Golden Dawn Enochian Skrying Tarot. It's perfect for advanced magickal divinations and works with powerful Enochian magick concepts; it also moves the Golden Dawn into the 21st century with new, but traditional directions.