The ultimate cause of the Chernobyl accident was similar to this. The reactor design was one which, if you put in fuel rods containing depleted uranium, could be used to make plutonium. Plutonium is military stuff that goes bang, therefore methods of making it are secret and everything around secret stuff has to be secret, or so goes military doctrine.
One of the things that was made secret was an annoying little misfeature of the reactor design. The core was graphite and graphite expands when hot. The control rods were engineering fit rods, and as such tended to stick a little when the reactor core was hot and had expanded a little, thus making the fit a little bit tighter. To solve this, the first eight inches or so of each control rod was graphite, which as well as a nuclear moderator is also a good lubricant.
The problem with all of this was simple: graphite is a moderator; stick graphite into what was an empty hole and the reactor initially starts to run even better than before, until the rest of the rod descends into place and the cadmium starts absorbing neutrons and shutting down the nuclear reaction.
Standard "Oh golly, shutdown" procedure with a nuclear reactor is to drop control rods into it as fast as possible, even all at once to shut down the chain reaction FAST. Do that with this sort of reactor and you suddenly make it run a lot lot better which makes the core hotter which jams the control rods at that point, and shortly afterwards the whole thing goes seriously bang.
Correct shutdown procedure here is firstly keep all coolant systems running full belt, and secondly insert rods quickly but sequentially. You don't shutdown the reaction quite as fast, but you do live to tell the tale.
All because that little foible was a military secret that the civilian plant operators weren't cleared to know.