Thirty years ago, the staff running a test on reactor #4 at the Lenin Nuclear Power Plant near Pripyat, Ukraine, USSR were reading unexpectedly high radiation levels. They debated stopping the test, but decided to keep going to find the limits. When the temperature readings climbed too high as well, they tried to shut the reactor down by inserting carbon rods.
There was, however, a design flaw, known by upper levels in the government, but not by the staff doing the testing. Inserting those rods somehow increased the reaction, increasing the heat. Containment water became steam, the roof of the reactor blew off and some 12 tons of radioactive uranium became airborne, contaminating a large swath of Europe.