Most expensive metric-imperial conversion error

On 23 September 1999 NASA’s ambitious Mars Climate Orbiter mission passed behind the planet Mars during its orbital insertion manoeuvre, after a nine-month journey to the red planet. Communications were never re-established and an investigation revealed the spacecraft had disintegrated in the Martian atmosphere due to human error. The onboard software had been written using metric units of thrust, while imperial units had been used to enter course correction commands from the ground. The Mars Climate Orbiter mission cost $327.6 million.