I found this in one of the books I am reading and it hit me, I learned a lot from the first few pages and thought that I should share a small part of it.
"The casting of spells involves working with powerful (and
often dangerous) magickal energies and is by no means something
that should be undertaken by an untrained novice.
Whenever working with energies, you should always take care
to protect yourself the best you can through the use of magick
circles, amulets, talismans, and so forth. You should also be
warned that, despite your magickal knowledge and your best
efforts, the possibility of any kind of a spell backfiring always
exists. This is not an uncommon thing to have happen, and
many of the practitioners that I know, including myself, have
experienced it at least once. It has nothing to do with karma,
displeased gods, or Gerald Gardners threefold law, despite
what some people choose, or are led, to believe. It has everything to do with the instability of magickal energy and/or a
practitioners incorrect application of it."
A lot of people on here mostly think the main reason why a spell backfires is because of the threefold law, well that might be the case but if you look deeper into the matter then you will find out the real reason.
*book: Herbal Magick a witchs guide to herbal enchantment folklore and divina*