Lord Acton hit the nail on the head. 128 years ago he wrote that, ""Power tends to corrupt, and absolute power corrupts absolutely". Unfortunately, no one has ever devised a way of running societies without giving power to certain individuals. With a very small number of honourable exceptions (which prove the rule), that power has corrupted them. We see it around us almost every day, and the people who rise to control entire nation states display the corruption due to power in singularly pure and concentrated form.
The technicians working in the FBI labs had a very limited form of power, but within that particular domain their sway was almost unchallenged. What expert would dispute the word of the mighty FBI, what lawyer would challenge it, what judge or jury would not be impressed by it? And the technicians' bosses had more power, which was assuredly focused on the important task of getting convictions. I rather doubt that any lab technician at the FBI ever got much career advancement out of frequently discrediting prosecution evidence. Every bureaucratic organization measures itself according to a limited set of drastically oversimplified metrics, and conviction rate is an important metric for any law-enforcement organization. The higher up the tree you go, the greater the desire for more convictions, and the less the concern for whether they are justified or not.
"One of the many reasons for the bewildering and tragic character of human existence is the fact that social organization is at once necessary and fatal. Men are forever creating such organizations for their own convenience and forever finding themselves the victims of their home-made monsters".
- Aldous Huxley