Eyewitnesses are unreliable with a frightening degree of regularity. They often make mistakes in their testimonies or claim to have seen things that it is later proven they never could have seen. We know more now than ever before about how often they’re wrong since we have DNA evidence. It has overturned hundreds of convictions, many based on eyewitnesses claiming to have seen a certain person — a person who, the DNA later showed, was never there at all.

This happens for many reasons. Lighting conditions. Bias. Distance. The chaotic nature of a crime scene. In general, a person’s memory simply is not as reliable or static as they think it is.

One major reason to consider, though, is known as the weapon focus effect. Researchers noticed that crimes involving weapons tended to have more issues with inaccurate testimonies than those without. They wondered why these witnesses were wrong so often.

As it turned out, the reason is that the witnesses tend to focus not on the person but on the weapon itself. It’s driven by fear. In some cases, people can barely describe the perpetrator at all, but they can absolutely describe the knife or the gun used in the attack. They may then later pick someone out of a lineup who somewhat resembles the person who committed the attack, but they’re far less certain about it. Despite spending so much time with a clear view of the person’s face, they can’t remember what they looked like.

The unfortunate reality is that witness mistakes lead to false accusations. They put innocent people behind bars. That’s why it’s so critical for everyone to know their rights and legal defense options.