Unfortunately, some did know just how possible it really was, but that information and the inherent risk of graphite-tipped control rods was considered a state secret and kept from those who unknowingly kicked off Chernobyl.

As wreckless as Anatoly Dyatlov seemingly was, he very much believed he had a "Get Out of Jail Free" card in the form of the AZ5 button, which when pressed would immediately dump all control rods into the core, theoretically at least disrupting the reaction by inhibiting the circulation of neutrons. Unfortunately because the control rods had graphite tips, when the control rods are first inserted, the initial reaction is actually enhanced, and in the case of Chernobyl, took it over the tipping point.

There were in fact multiple "mini-Chernobyls" which happened prior to the "big one", the largest of which was an incident in 1975 at the Leningrad Nuclear Power Plant.

I find this stuff fascinating, and if you're interested in learning more (as I was), take a look at the book "Midnight in Chernobyl", or if you're more inclined, listen to the book "Chernobyl- History of a Tragedy Part 1" on Youtube.