I'm not skilled in any way but I used to be allowed to drive a 'Live Steam' model as a teen so I'll start things off with:
Water management in the boiler. If you let the crown of the firebox become uncovered you have what is now potentially a very large bomb on your hands. Even if you don't kill anyone you have very likely taken the locomotive out of service for years due to damage to the firebox crown. However if you mistime topping the boiler up with water you can reduce steam production because you are adding cold water to hot, which leads to less power, which you might need.
This is what can happen:
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Look at this video. It shows what happens when an inexperienced crew overfills the boiler, then allows wheelslip to develop, followed by mishandling the Regulator so water is carried over into the Regulator Valve ('priming'), meaning that the Regulator couldn't be closed. When the driver tried to bring the Reverser into Neutral to deal with this that in turn ran away into full forward gear, injuring him. The engine was then stuck in a full forward power wheelslip until the cylinders and motion all came apart.
https://youtu.be/YjsNbzg1UaI
Finally, get it wrong with the fire and the Regulator (or a few other things wrong) when going under a bridge or into a tunnel and you can get a 'Blowback'. Its a bit like what firefighters call a 'flashover' and can fill the cab with flames. The consequences can be very serious, In 1965 Driver Wallace Arnold Oakes received the George Cross posthumously after he received fatal burns whilst remaining at his post to close the Regulator and apply the brake to bring his express passenger train to a stand after a blowback.
PS: More qualified people, please comment!
I don't know where to start with that, but you've half understood and mixed up practically everything you've said.
What I will correct is that Dvr Oakes was driving a loco that suffered a burst pipe union to the steam brake valve in the cab. This is an approx. one inch pipe, that will spew boiler pressure steam into the cab. The regulator valve was found open, as he couldn't get near it to shut it. RIP.