Human error
Days, weeks, month, or years after a disaster, it is often easy to look back and see where thing went wrong. This is perhaps even even more true of disasters that are cause by human error - usually small mistakes with very big results.
The Great Fire of London in 1666 started as a small kitchen fire, but its effects changed London for ever. The Titanic and the Exxon Valdez disasters happened at sea. Both were caused by an unbelievable confidence that nothing could go wrong, and that confidence resulted in serious mistakes which are still discussed today. The Bhopal and Chernobyl disasters resulted from scientific experiments that went badly wrong.