In some cases, when people are pulled over for driving under the influence of alcohol, they will be surprised to fail the breath test. Even if that person knows that they have consumed alcohol, they will honestly say they did not feel very intoxicated. They thought the BAC results would be low, and they did not think there was much of a chance that they would receive a DUI.
Part of the reason why this happens is that people are notoriously bad at judging their own intoxication. Often, the way they do it is simply by comparing themselves to those around them. This can lead to many inaccurate beliefs, depending on the setting and how much other people have been drinking.
How this could make a difference
For example, imagine that a college student turns 21 and goes out to the bar to drink with their friends. They have three drinks, but everyone in the bar has had far more than that. Maybe most of the people in their group have five or six drinks in the same timeframe. The college student feels relatively sober.
On the other hand, imagine that the same college student has to take an exam, but they forget about it. They have had three drinks at their apartment when they remember the exam, and they rush to campus to take it. If they are sitting in the exam room after having the exact same three drinks, they may feel heavily intoxicated compared to the sober and quiet students around them.
When people misjudge their own intoxication, it can lead to serious criminal charges, and it is very important for those individuals to understand what legal options they have moving forward.