Witchipedia

I do not know what you mean by "backfire", but I'll make a go of it.
A spell is the manipulation of subtle energies in order to bring about a result. If done correctly, the energy will flow toward the desired outcome using the simplest possible route- that with the least or the most easily overcome obstacles. If you are concerned that the easiest possible route would lead to an undesirable outcome, you must either find a way to avoid that outcome or simply not do the spell. Spending some time brainstorming the possibilities before casting a spell is always a good idea. For example: the easiest possible route to money might be an inheritance, which would, of course, require a death. If that's not acceptable to you, you need to take steps to avoid it. The fix here, beyond, of course, not doing the spell, would be to specify where the money would come from- earnings, for example, keeping in mind that the easiest possible route to "money from earnings" might be the offer of a job that really, really sucks. That's just how magick, indeed life, works. And the more specific you get about the acceptable routes, the harder the easiest possible route becomes and so it might take more time to accomplish. A spell for money, from income from an enjoyable job that doesn't result from anyone's misfortune will probably take longer to see results from than just a spell for money, leaving all possibilities open because it leaves fewer cracks for that energy to flow into.
It's all metaphor of course, but I hope it helped.