I was a juror on a criminal trial once and there was a ballistics expert who testified that he was "100% certain" that the markings on a shell casing came from

I was a juror on a criminal trial once and there was a ballistics expert who testified that he was "100% certain" that the markings on a shell casing came from a certain gun. This is based on a visual inspection under a microscope. I googled it after the trial and sure enough when you put this sort of thing to the test, experts are not correct 100% of the time.