Of the 367 wrongful convictions that were exonerated since 1989 by the Innocence Project, 71% involved eyewitenss misidentification.
Supposed human memory every often is distorted and biased due to multiple factors.
Here's an interesting video dealing with the topic.
What can be done to mitigate the possibility of witness misidentification? There are many things. To learn about them, click here.
Here's part of what is stated on the Innocence project web site about police and prosecutorial reforms when it comes to using eyewitness testimony in prosecutions:
How to Improve the Accuracy of Eyewitness Identifications