The idea that spells can "backfire" seems really to be two ideas under the same name.
1) The spell you cast will cause the opposite result than the one you hoped for (the person you want to have love you will hate you; instead of the job promotion you hoped to get, you will be fired from the job)
2) The spell you cast will somehow turn into a sort of tar baby that will ensnare and entrap you in its clutches (instead of the person you want loving you more, you will love them more; instead of the job promotion you hoped to get, you will be forced to cast spells over and over to stop people on the job from envying your success).
In the first group we find backfiring spells that are said to be the result of casting spells "the wrong way" or with "the wrong words" or with "bad intentions." In the second group we find certain spells deemed to be ipso facto tar-babies and "dangerous" to the practitioner.
Both of these forms of backfiring are dramatic and fictional in their origins. Like the related concept of "spell overload" mentioned above, they are crippling to the practitioner, and may even have been designed by their creators to function in that way, as warnings about the "dangers" of practicing powerful folk magic outside the confines of sanctioned churchly miracles and blessings.
At the Lucky Mojo Forum, people ask me about "backfiring" all the time, and offer their opinions about it to other posters as well. Here are some quotes from the Forum, gathered during 2009 and 2010:
• "Too much work on a target, especially working on the mind, can backfire on you."
• "If you do mind reading spells wrong or for malicious reasons then they can backfire."
• "Bragging about magical successes can cause jealousy, strife, and definitely backfire."
• "Too often people get really forceful and that can backfire."
• "The target putting the oil on himself could backfire."
• "Intranquility spells can backfire on you, so be warned."
• "Spells for intranquility and separation can backfire."
• "It is not something to be taken on lightly and can definitely backfire."
• "Do you think that would work or could that backfire?"
• "If you do not do the bath PERFECT IN EVERY WAY does that mean it can backfire on you in a negative way?"
• "If I decide to hotfoot him, might it not backfire?"
• "I'm afraid it would backfire."
• "I just don't want to put in the wrong wordings and backfire instead!"
• "I don't want to try and kiss him and have it backfire."
• "I don't want to do anything that would be risky or could backfire in any way!"
• "I don't want the spell to backfire.
Do you see how self-defeating, self-scaring, and pointlessly dramatic those ideas are?
They are obviously the scripts of people who were raised to fear and avoid magic.
If you feel that your work will backfire, magic is not something to which you are habituated or for which you are gifted. Go do something else, something in which you have hopeful and cheerful confidence. PLEASE.