Words you do not want to be in a FDA report about your vaccine manufacturing facility (emphasis mine):
On March 26, 2021, Janssen notified FDA that they had detected AZ COVID-19 Vaccine virus in the Janssen COVID-19 Vaccine DS batch 21003600 (GMP8). This batch was produced during a period when Emergent implemented measures to handle increased waste production.
FDA subsequently engaged actively with Emergent and Janssen to facilitate the investigation of the root cause of this contamination event. It was concluded that most probable contributing root cause was that the bioreactor media prepared for use in the cell expansion process at that time was contaminated in the common weigh and dispense area through contact with the waste path for materials from Area 3 (AZ manufacturing area).
One of the weirder books I have seen in my life (not that weird really) is about GMP design of manufacturing facilities, and when I flipped through the book, there was a section about how you want products to flow in a specific direction, and for there not to be any cross-contamination. Now I guess we know why.