Forensic science encompasses a broad range of things, including bullet-lead analysis, arson investigation, handwriting analysis, fingerprinting, etc. Various techniques have bordered or even crossed into pseudoscience, others have wildly erratic error bars that are never properly considered. There are so many problems because the entire field does not treat their discipline as scientists do, they are too close to law enforcement for that. Techniques are used for decades before anybody bothers to properly figure out if the technique actually works.