* The Attack: Gandalf, along with Thorin and company, encounters a pack of wolves while escaping the goblins. Gandalf uses fire magic to drive them back, but some of the wolves survive and flee.
* The Consequence: The surviving wolves end up encountering the giant spiders of Mirkwood. Instead of attacking, they become the spiders' prey.
* The Unintended Impact: The wolves' loss to the spiders means that the spiders are left with no natural predator, allowing them to grow stronger and more numerous, becoming a danger to travelers in the future.
While Gandalf's magic initially helps the dwarves escape the wolves, the wolves' survival and subsequent encounter with the spiders ultimately leads to a bigger threat for those who venture into Mirkwood. It's not a direct backfire, but a ripple effect with unforeseen consequences.