Why would you think that? It's a truism; not even really debateable. Memory is notoriously unreliable. Words, faces, colors, all change. Countless studies confirm this.
Think about it in terms of deciding a case. For example, look at the difference between Ray Rice and Greg Hardy. With Rice, we know what happened. We may not know what led up to it, but we know he clocked that girl hard without much, if any physical provocation.
With Hardy, we have two competing versions of events based on the memories of intoxicated people with adverse interests that we characterize and envision in our own minds based on our imaginations and ideas about how the world works.
Which one are you more confident of knowing what really happened?