Which of the following is a common mistake made during surveillance?

Assuming the subject will exit through the same door is a common mistake made during surveillance because it reflects a lack of flexibility and adaptability to the subject’s behavior. In many situations, individuals may have multiple exits or may choose to leave a location through different routes to evade detection. A successful surveillance operation requires investigators to anticipate various possibilities and be prepared to adapt their approach if the subject changes their plans. Placing too much reliance on one assumption can lead to missing critical moments of a subject's movement, which could compromise the effectiveness of the entire surveillance effort.

Other choices, while they may also reflect potential oversights, do not capture this specific behavioral adaptability aspect as effectively. For instance, assuming the use of public transportation or planning to follow on foot can lead to logistical challenges, but they do not directly address how a subject’s choice of exit is often unpredictable. Lastly, staying in open areas for better visibility can be a tactic that may compromise the investigator's cover rather than ensuring they are adequately positioned to observe the subject without being detected.