No idea about the most important, but one US law that had massive (and apparently unintended) economic consequences is the IGRA (Indian Gaming Regulatory Act)?

No idea about the most important, but one US law that had massive (and apparently unintended) economic consequences is the IGRA (Indian Gaming Regulatory Act)? not a lawyer, but I believe this Act was the legal catalyst for the proliferation of casinos on Native American tribal lands ("reservations")