It is a well-known point of law that ignorance of a law does not excuse a person who has broken it. However, breaking a law in one jurisdiction by engaging in activity that's legal in another can happen with relative ease. A recent example in the news comes from a California man facing drug charges here in Texas for possessing medical marijuana.
On Jan. 1, a 67-year-old resident of California was stopped by police in Decatur for allegedly speeding while driving to Houston to visit his granddaughter in a hospital. The man admitted to the officer that he was in possession of a pipe, but that he had a prescription from his doctor for marijuana for treatment of pain and other side effects from a variety of medical issues, including bypass surgery and polio. He claims to have been using prescription marijuana for 10 years.