It was part of my training for nuclear industry public relations work. There was a particularly nasty one I recall where several men received lethal radiation doses during a criticality accident while arranging plutonium slugs for transport.

Eventually, a device called a "birdcage" was introduced. It consisted of a framework of aluminum where the plutonium slugs could be loaded. The geometry was calculated so that even when fully loaded with plutonium, sub-critical masses of the material could be packed in such a way not enough of it was in sufficient proximity to any more for a criticality excursion to take place.

These incidents were declassified after the war, they had to be made public so the new nuke industry could develop appropriate safety standards and procedures. Of course, care was taken not to publicize them too much.