Yes, they certainly can backfire.
Many years ago I had one backfire on me. I had done a spell to bring some needed money into my life. A few days later I was involved in a car accident. Three months later the insurance company sent me a check which paid for the damage to my car, my medical bills, and some extra. Not exactly what I had in mind when I cast the spell, but the end result was what I asked for.
I've also encountered people who have done money spells only to have someone they cared about die and leave them money in their will.
Of course this isn't what happens in every case, but money spells can and do go awry. That's why your visualization and intent have to be very clear when casting.