I always thought signs that said "no trespassing" were kind of funny. Since trespassing is by definition illegal, the sign is like "no stealing" or "no murder."

I also love the printing on software CD ROMs: "do not make illegal copies of this disk." Is this necessary? Doesn't everybody know that a software company doesn't want them to make illegal copies? What do they expect in response? "Well, I was all set to break the law and defy the authorities, but since a software company said not to..."