Ariane 5 exploded 37 seconds after launch because of a software overflow error

On June 4, 1996, the maiden flight of Ariane 5 ended 37 seconds after liftoff. A software exception in the inertial reference system fed diagnostic data to the flight computer as if it were flight data; the computer obeyed, swiveling the engine nozzles hard over. The vehicle pitched past 20 degrees of attack, aerodynamic loads tore the boosters loose, and the automatic self-destruct fired — destroying the launcher and its four Cluster science satellites.

The root cause was traced to a piece of software reused from the older Ariane 4 system. During the first phase of flight, the Horizontal Velocity Bias (BV) was significantly higher than on Ariane 4. The code attempted to convert this 64-bit floating-point number into a 16-bit signed integer. Because the value exceeded the maximum limit for a 16-bit integer, an arithmetic overflow occurred.

The cruelest detail: the overflowing routine was an alignment function only needed before liftoff. It had been left running into flight purely because it had always run on Ariane 4 — where the rocket's gentler trajectory kept the value small enough to fit. Both redundant inertial units ran the identical code, so the backup failed the same millisecond as the primary.