From Hal Arkowitz and Scott O. Lilienfeld, Why Science Tells Us Not to Rely on Eyewitness Accounts, from the January 2010 Scientific American Mind:
The uncritical acceptance of eyewitness accounts may stem from a popular misconception of how memory works.
Many people believe that human memory works like a video recorder: the mind records events and then, on cue, plays back an exact replica of them.
On the contrary, psychologists have found that memories are reconstructed rather than played back each time we recall them.
The act of remembering, says eminent memory researcher and psychologist Elizabeth F. Loftus of the University of California, Irvine, is “more akin to putting puzzle pieces together than retrieving a video recording.”
Even questioning ... can alter the witness’s testimony because fragments of the memory may unknowingly be combined with information provided by the questioner, leading to inaccurate recall.
(Extra paragraphing added.)