The Soviet Union enacted an infamous law in 1922 that
criminalized "hooliganism.” The crime was in the eye of the
beholder, the beholder of consequence being the Soviet secret
police. Because it was impossible for dissidents to know in advance
whether they were violating this prohibition, they were always
subject to arrest and imprisonment, all ostensibly according to
law.



In the United States, we have legal safeguards against
Soviet-style social controls, not least of which is the judicial
branch’s ability to nullify laws so vague that they violate the
right to due process. Yet far too many federal laws leave citizens
unsure about the line between legal and illegal conduct, punishing
incorrect guesses with imprisonment. The average working American
adult, going about his or her normal life, commits several arguable
federal felonies a day without even realizing it. Entire lives can
change based on the attention of a creative federal prosecutor
interpreting vague criminal laws.



http://reason.com/archives/2011/06/21/what-you-dont-know-can-hurt-yo