The RBMK reactor had a known design flaw kept secret from operators

Higginbotham details how the RBMK reactor type used at Chernobyl had a dangerous quirk: under certain low-power conditions, pushing the emergency shutdown control rods in could momentarily spike reactivity instead of suppressing it, due to graphite tips on the rods that displaced water before the absorbing material engaged. Soviet nuclear engineers had encountered warning signs of this flaw in earlier incidents at other plants, but the information was classified rather than distributed to control-room operators who needed it most.

The operators running the ill-fated test on the night of April 26, 1986 pressed the shutdown button believing it would stop the reaction, unaware it could briefly do the opposite under the plant's unusual configuration at that moment. This wasn't just a control-room error; it was decades of institutional secrecy denying frontline workers the exact knowledge that might have prevented catastrophe. Takeaway: a safety flaw that's classified rather than communicated is functionally the same as no fix at all.