From Reason.com:

When Joseph Cracco bought his folding knife about a decade ago, he had no reason to think he was breaking the law. Nor did Damien Guedes when he bought a bump stock for his AR-15 rifle in 2014.
Both products were subsequently transformed into contraband—not by legislators but by law enforcement officials. The way that happened shows how easily policies aimed at promoting public safety can make all of us less secure by undermining the rule of law.