Just watch the History channel, it's on every couple months or so. It basically came down to running the generator with all security features being turned off, on purpose, not because of an accident. They were trying to test how slow the reaction can be kept going (apparently nuclear reactors do not perform well running slow) but all the security features were trying to shut it down so they kept disabling them until the reactor exploded. Three mile island is much scarier since there the monitoring equipment failed (and wasn't monitoring all that it should have).

Jerry:
It was not for a test. It was for optimizations of power generation.
In case of stable system - turbines working at constant speed - you can get only some theoretical max % from power generated by nuclear reaction. But in case if turbines working at different speeds - once go to high speed and then giving away it's kinetic energy you can yield better % from nuclear reaction.
For this reason there was needs to slow down nuclear reaction and then get it back, slow down and get it back, and so on.... But safety measures was designed to not allow nuclear reaction to increase power too fast.
As result safety was disabled to allow turbines increase their speed and this resulted people was unable to shop reaction (the same operation that disabled safety, also disabled emergency stop button) at needed time - it blow up.

I've read about this in a Russian technical magazine "Science and Life".

This 3-5% optimization killed a lot of people :o(

A different opinion exists in public and I'm unsure which one is correct.
http://www.world-nuclear.org/info/chernobyl/inf07app.htm

Mine is possibly flawed as I do not know how big is kinetic energy of rotating turbines and how it can be compared to overall generated.

FYI, I've been told that there is a debate regarding the authenicity of the "Kid of Speed" travelogue. Nothing definitive, but FYI.