here's a piece on it at High Road.
Short version: FBI for years said their lab could analyze the bullet lead or fragments at a crime scene, compare them to bullets in a box owned by a suspect and declare "This bullet did/did not come from this box."
Until it was proven that this was bullcrap. Now a bunch of cases are under review, and probably a lot more should be.
You think about it, this should have been questioned from the beginning. Ammo companies get lead by the ton daily; being honest, all you could say was "This bullet came from the same batch of lead as was used in making this box of cartridges. Along with about a million others." But that doesn't impress people with your EffingBI Lab wizardry and put people in prison.