Hypothetical: Suppose you are stopped under suspicion of driving under the influence of alcohol, and your blood alcohol content registers at .07. Under the law, the legal limit for drunk driving is .08. A few weeks later the legislature passes a new law lowering the threshold to .05. Shortly thereafter you receive a summons in the mail charging you with drunk driving under the new law. Can you be convicted?
As a general observation, the answer is no. Our constitution prohibits the enforcement of ex post facto laws. From a due process standpoint, citizens have a right to know in advance the legal standards they will be held to. To be charged under an ex post facto law is fundamentally unfair, amounting to changing the rules in the middle of the game.
— Kevin Palmer