Final answer:
Mary's inability to recall the assailant's appearance but detailed memory of the gun is an example of the weapon focus effect, which draws a witness's attention to a weapon, impairing their ability to remember other important details of the event.
Step-by-step explanation:
Mary's inability to describe the assailant, while being able to vividly recall the gun, could be due to the psychological phenomena known as weapon focus effect. This occurs when the presence of a weapon in a violent crime attracts the witness's attention, hence, they tend to focus on the weapon itself instead of other details like the assailant's appearance. The weapon focus effect can impair the witness's ability to accurately recall and identify the perpetrator because the weapon draws their attention and cognitive resources at the expense of other details.
The memory errors such as absentmindedness and blocking are also common issues that could affect a witness's recall ability, but these do not specifically address Mary's scenario, which is closely related to the intense focus on a weapon during a stressful event. It is essential for law enforcement and the legal system to be aware of these limitations in eyewitness testimony to prevent identification errors and ensure that justice is served correctly.