Even the most honest, credible and sincere witnesses can be mistaken about their facts.
I witnessed a deadly car accident. I was a witness in a civil trial, in a lawsuit over the death.
I was right behind the car, in which the passenger died. We were in the left lane at a busy intersection, on the Pacific Coast Highway, waiting for breaks in traffic, to turn left. The car in front of me turned left right in front of a fast moving car, and was T-boned feet away from me. It was so frightening.
I saw it all and was totally honest with everything I saw. I was interviewed on scene, and then went to a deposition and testified in court.
I was absolutely certain that everything happened the way I saw it and remembered it. But I was shocked when I saw the surveillance footage from the bank on the corner---and a couple of things were not as I remembered them. And I was so certain about what I 'thought' I remembered.