That's what a Montana woman alleges happened to her in 2010. Leigh Paffhausen claims that she was given a small amount of GHB (also known as the "date-rape drug") in her drink, which caused her to drive erratically and get stopped. But when she tried to raise that issue as part of an "involuntary intoxication" defense, the state denied her the opportunity. Thankfully, the Montana Supreme Court granted her appeal and ruled in her favor, sending the case back to the lower court.
Prosecutors (and three of the seven dissenting judges) claim that this ruling will lead to a flood of "somebody slipped something in my drink" defenses. But this is a silly position to take for two reasons. First, it's disingenuous for a prosecutor to argue in one courtroom that a person raped someone because the victim was powerless due to a date-rape drug, then walk to another courtroom and prevent an accused drunk driver of claiming that he or she was powerless for the same reason. But more importantly, Paffhausen (and anyone else) still has to prove that she was slipped the drug. And that's far from an automatic "get out of jail free" card.
Finally, it's the prosecution's job to seek the truth! Rape for example is a class A Felony that would be considered by many a more important crime to prosecute than DUI, a gross misdemeanor. Additionally, a basic element of any criminal act, including DUI, is volition or consciousness. If someone is has no will to commit the crime because they aren't aware that they are doing it they are not guilty and shouldn't be prosecuted.
It won't happen here in Seattle, Washington you say? The prosecutors are moderate with their power and will seek the truth. I'm sad to say that we just finished defending a woman in the same shoes as this Montana woman.