I was curious too. This is my best guess,
https://en.wikipedia.org/wiki/Chernobyl_disaster
The reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/h), while the true levels were vastly higher in some areas.
Because of inaccurate low readings, reactor crew chief Aleksandr Akimov assumed that the reactor was intact. The evidence of pieces of graphite and reactor fuel lying around the building was ignored, and the readings of another dosimeter brought in by 04:30 were dismissed under the assumption that the new dosimeter must have been defective.
Note the name Akimov in the post.











Even better is when the parents do it. I wasn’t present, but the conversation was about my parents’ marriage, my sister-in-law did the mathing and realized my birth date (I’m the first child) and their marriage date were less than 9 months apart. In my family and social circle, sex out of wedlock is quite looked down on. When she vocalized this, the subject was changed very quickly and brushed aside.