Good fellow moves to Mass., complies with gun laws, winds up sentenced to a year
Meet Clint Cornelius. He decides to move to Northampton, MA. He researches MA gun laws, and notes that a person moving there has sixty days to register guns and comply with its laws (which also restrict ammunition). He transports his three firearms in accord with MA law.
But he didn't realize that MA also has a ban on large capacity magazines, and that is NOT subject to the sixty day window. The firearms are, the ammo is, but magazines are not. So he winds up serving a year in jail for having three, unloaded, pieces of metal. And he only got that by taking a plea -- if convicted of having the pieces of metal, he'd have gotten a mandatory minimum of 2.5 years in prison!