The welds in the bodies of the earliest-built cars did not achieve full penetration of the material. Strength-wise, they are more than adequate and safe, but it does allow for the potential for the joint to fatigue over time and eventually fail. And to be fair to the workers, this is generally type of thing that only gets discovered by fairly advanced processes, not by looking at it closely with a skilled eye.
The damning thing in this for Bombardier is that the "fairly advanced processes" are supposed to be methodically and diligently applied in qc'ing the fabrication. How production reached 60+ cars before this became an issue just astounds me. The techniques may be complex, but the principle of QC'ing welds thoroughly is Quality Management 101 material.