Here's an interesting immigration tidbit: Section 264(e) of the Immigration and Nationality Act states:

Every alien, eighteen years of age and over, shall at all times carry with him and have in his personal possession any certificate of alien registration or alien registration receipt card issued to him . . . [this means the actual green card, not a copy]. Any alien who fails to comply with the provisions of this subsection shall be guilty of a misdemeanor and shall upon conviction for each offense be fined not to exceed $100 or be imprisoned not more than thirty days, or both.