When you are pulled over by the police and they believe that you have been driving while intoxicated, they will go through a number of steps to determine your inebriation. There's your standard field sobriety test, which involves an officer asking you to do a number of tasks. There is a breath test, which can be done by Breathalyzer. A blood or urine test may be required later.
But all of these tests have faults, may it be by the way the test is performed by an officer, or by the way the evidence is handled thereafter. Let's start with the first test, your standard field sobriety test.
The officer could ask you to perform a number of actions here, such as saying the alphabet backwards, walking in a straight line toe-to-toe, and touching your nose when your arms are initially extended outwards from your body. All of these actions are difficult for a sober person to pull off without the stress of a police officer watching them, let alone a possibly intoxicated person during a traffic stop. These tests have been ridiculed by many as not providing any proof of a person's sobriety or intoxication.
The breath test is a big one in the state of Minnesota. Under implied consent, you have to take a breath, blood or urine test to establish your blood alcohol level, though you can refuse an initial field sobriety test. However, the breath test could be improperly administered by the police, invalidating that evidence.
Under the same premise, the evidence garnered from a blood test could be mishandled or tainted. The test itself could even be botched by a lab technician.
Source: FindLaw, "Sobriety Tests," Accessed Oct. 23, 2014