Laws cannot be unconstitutionally vague or ambiguous
In order for you to be guilty of a crime, you have to violate a law that is not unconstitutionally vague. Sometimes, state legislators pass laws that are too vague or ambiguous to be legally enforceable. In such situations, if the law is struck down by your appellate courts, this frees you of all charges, since the law under which you were arrested was not constitutional.
As an example, the Georgia legislature passed a law making it unlawful to drive while you had any marijuana in your system at all. The courts found this unconstitutional because no connection existed between the very low levels of any legally used marijuana and any concerns for driver safety and protecting the public. In Texas, where a motor vehicle law was written in such a complicated fashion as to not be able to be understood, it was declared unconstitutional. The wording of the statute was "forbidding the operating or driving of motor vehicles on any public highway 'where the territory contiguous thereto is closely built up, at greater rate of speed than 18 miles per hour,' was too indefinite and ambiguous as to the places where it is to apply to be operative." On the other hand, just because "driving under the influence" is defined in two different ways by statute in many states, this does not make this term vague and thus unconstitutional.
If the law you were charged under is later declared unconstitutional, your attorney can use this as a defense
If you have been charged under a DUI-DWI statute or ordinance that is later declared unconstitutional or otherwise invalid, or has already been declared unconstitutional at the time of your arrest, your attorney can raise the defense that you had not in fact violated the law, as the law itself was not valid. Although such challenges are rare, top criminal defense attorneys raise such challenges when it may win your DUI-DWI case. Recent aggressive law enforcement activity has increased the chances of unconstitutional acts occurring in the effort to arrest more "drunk drivers." http://www.duiblog.com/2005/01/09#a82.