1) Odor of Alcohol
Turning the mere odor of alcoholic beverage and using it unfairly to presume intoxication. There are many factors that may contribute to the odor of alcohol, including mouthwash, denture solutions, having an alcoholic drink accidentally spilled on one's clothes at a party, consumption of alcohol by other passengers in the vehicle, or minimal consumption (1 or 2 drinks), etc.
2) Other Reasons for Bad Driving
Assuming that a specific driving behavior is related to alcohol consumption. Many things can contribute to why someone may exhibit unusual driving behavior, including "texting while driving" and talking on a cellphone. Many times people weave due to reaching for cell phone they drop on the floor. Often officers will use this and turn it into evidence of intoxication.
3) Medical Conditions
Many medical issues can mimic the clues of intoxication that investigating officers are trained to look for. Often the medical condition may be too personal to discuss with the officer. Or perhaps the driver is too nervous to mention what the medical condition is. Officers almost always fail to explain how past medical issues that you no longer consider a disability can cause you to fail to perform the test to their satisfaction.
4) Physical Inabilities
Often people are phyically not candidates for the SFST's. People who are overweight and/or over a certain age are not candidates for the SFST's, although officers routinely tend to ignore this factor. Often old sports injuries may affect someone's ability to perform the tests. Many people cannot pass these exercises even while totally sober. The officer has no comparison of how you would perform the same test on your best day, let alone while you are pulled over and demanded to perform the tests on command.
5) Improper Instructions/Demonstration of Roadside Tests
Often the investigating officers will make mistakes when giving the instructions and when demonstrating the SFST's. The Officer must give proper instructions and demonstrate the test to the citizen in the correct manner before the Walk and Turn Test and One-Leg Stand Test can be performed. NHTSA clearly states: "If any one of the SFST elements is changed, the validity of the test is compromised!" Officers will later say that they "substantially complied" with their instruction/demonstration, even though any deviation on your part result will result in a failure. If the instructions or demonstration is done incorrectly by the officer then the results cannot legitimately be used against a driver.
6) Distractions During Roadside Tests
Sometimes there are distractions which will impede the performance on the SFST's. Officers usually are aware of, but rarely eliminate, distractions during SFSTs. Some distractions could include loose gravel, uneven surfaces, environmental issues (rain, mist, wind, bad lighting), flashing patrol lights, etc. These factors and others can prove very problematic when performing the SFST's, although officers will not take these into consideration when determining if you will be arrested for DWI.
7) Improper Sequence of Instructions
Often the investigating officers will not follow the proper sequence of the SFST instructions according to the NHTSA Manual. Again, NHTSA clearly states: "If any of the SFST elements are changed, the validity is compromised! Officers will often forget an instruction, then add it in later out of order. The officer cannot just make up the instructions in whatever manner they choose. NHTSA has very strict requirements for the performance of SFST's, and they should be followed to the letter.
8) Utilizing "Invalid" Tests
Officers often use invalid "tests" which are not supported by the NTHSA research studies to aid in their evaluation of intoxication. Any test not supported by the NHTSA research should not be used when making a determination of intoxication. The SFSTS's have enough potential rate for error by themselves. Therefore, other tests that have no basis for an evaluation of an alcohol intoxication level should not be used.
9) "I Am Just Going To See If You Are Okay To Drive Home..."
Investigating officers routinely trick the driver into trying to perform the SFST's under the false belief that they will be released if they pass the test. Not many people want to spend the night in jail and be arrested for DWI. As a result, many people end up trying to perform the tests because they believe that if they pass, they will be able to go home. The officers very commonly already have their mind made up, and they are just trying to encourage participation in the SFST's to help gather evidence to support the arrest.
10) Not Following Proper Procedure When Obtaining Chemical Specimen
The arresting officer must read the proper DIC paperwork to the driver when requesting a chemical specimen. Often the paperwork is read incorrectly, or outdated paperwork is used, or the paperwork is not read at all. Not following this procedure can render a chemical test result inadmissable. As well, proper protocol must be followed when the chemical result is obtained. Often corners are cut to save time, such as not following the required "15-minute observation period" when obtaining a breath specimen. In DWI blood test cases, the officer must adhere to all steps involved with obtaining a search warrant if they seek to draw the blood without consent (and assuming it is not a situation where a warrantless blood draw is allowed). There are many rules and regulations that the officers must follow in order to be able to use a chemical test in court, and often they are not.