He told me that overwhelmingly what went wrong at Chernobyl is that the head of the plant consistently ignored all of the alarms and monitoring in place, because he was overconfident at what the results of his test would be. He apparently died in the control room.
Interestingly, one thing the Russians did notably different was to have a big guy, I want to think at the sister site his name was Sergei, basically live in a hut next to the cooling lake, which was uphill from the plant. In the event of a disaster, he had a humongous mallet he would use to slam open the valve, draining the lake.
... or maybe this was all made up, I couldn't tell you tbh, but it sounded legit to me. ;)