Harry works best with WWII level tech or earlier. His car is a 1960's beetle and even that ends up in the shop constantly because he accidentally hexes it. It's not a hard and fast rule that you can graph though, and much of the effect depends on his emotional state, random chance, and the tech involved.
Also, as far as I know the Murphyonic Field (what ever can go wrong will go wrong) is an artifact of the human use of magic. I could be wrong, but that's what I've always thought.
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