For some insane reason, I've been binge reading books related to the Manhattan project. The takeaway is that much of the time they were flying blind or wearing blinders due to the extreme time pressure.
Sometimes they were plain reckless. The criticality experiments is a good case; adjusting the distance between two semi-critical cores with a screwdriver jammed in between. Great fun until the screwdriver slipped and the core went critical. Didn't go boom but killed a guy. For good measure they repeated the experiment and killed another guy.