An uninforceable law?
Why does that sound so familiar?
There are hundreds of such laws on the books, ranging from bizarre laws that were never intended to be enforced, to holdovers from an earlier time that are no longer legal or ethical to enforce. There are entire websites dedicated to both extremes.
It is important to note that while most unenforceable laws (such as prohibitions against fake moustaches in Alabama) will simply sit--unnoticed and ignored--on the law books, many unenforceable laws can be used as a blank check for police to bully seemingly law-abiding citizens. The most famous example was Lawrence v. Texas (2003) where police investigating a fraudulent domestic violence report stumbled upon two adult gay men in the act of sex. Embarrassed, they arrested and charged them with sodomy, which was common but technically illegal at that time. The case went to the Supreme Court, which declared all sodomy laws to be a violation of the Fourteenth Amendment. The case has recently brought attention to dozen of other laws across the country that were used by (usually fundamentalist) police or other officials to push around people that they are morally at odds with. Most notably, it is currently illegal in Florida, Mississippi, and Michigan to cohabitate with anyone except your lawful spouse.