In Australia at least, you need to maintain records showing testing of each batch and equipment calibration. Government auditors can come at anytime to review the records, and will shut things down if records are missing/altered/or lacking. Pharmaceutical manufacturers aren't constantly shutting down, so we can assume contamination is fairly low.

Just because people write a very low or zero value in the records doesn't mean that's actually true for the product that physically is consumed.

At best it can be said to imply what's likely the case in physical reality. But even assuming a very low corruption rate of 0.1%, that's still hundreds of bad record writers putting out bogus records.