My dad told me about an incident in the 1950's on an aircraft carrier where a disgruntled sailor flushed a pound of sodium metal down the toilets and blew them up, disabling them. Needless to say, the sailor was court-martialed, and my dad had a seat on the court. It seems that the key part of getting a major reaction from sodium metal and water is to constrain the reaction, much like the way that gunpowder will burn when unconstrained but explode when constrained.
The problem with Mythbusters is that they often don't think through how to really reproduce a situation and have terrible experimental procedures, such as when they used a bolt-action rifle to test the effects of blocking the barrel of a gun, rather than testing on a semi-automatic like a Colt .45 pistol (which was the original of the myth that they were trying to bust), or when they were testing whether on gets better fuel economy to driving with the windows down on a car or with the air conditioning on - they changed the conditions of the experiment halfway through on that one.
The problem with Mythbusters is that they often don't think through how to really reproduce a situation and have terrible experimental procedures, such as when they used a bolt-action rifle to test the effects of blocking the barrel of a gun, rather than testing on a semi-automatic like a Colt .45 pistol (which was the original of the myth that they were trying to bust), or when they were testing whether on gets better fuel economy to driving with the windows down on a car or with the air conditioning on - they changed the conditions of the experiment halfway through on that one.