On June 4, 1996, the first flight of the Ariane 5 rocket, designated 501, ended in disaster just 36.7 seconds after liftoff. The rocket broke apart and exploded, destroying four scientific satellites from the European Space Agency's Cluster mission. These satellites were designed to study the interaction between the Sun and Earth's magnetic field, but they were lost in the explosion. The failure was caused by a software error in the rocket's onboard computers, which were meant to act as backups for each other in case of a problem. However, both systems failed at the same time due to a data conversion error. The problem arose from a 16-bit overflow during a calculation that converted a 64-bit floating-point number into a signed 16-bit integer. This conversion was part of an alignment routine for the rocket's inertial platform, which was no longer needed after liftoff but had been left active for the first fifty seconds of flight. The value being converted was far higher than what the previous Ariane 4 rocket had ever produced, exceeding the capacity of the 16-bit register and triggering an error. Since both of the rocket's guidance systems used the same software and data, both systems encountered the same error within a few milliseconds of each other. As a result, the backup system failed in the same way as the main system, leaving the rocket without any functioning guidance. Without reliable data on the rocket's orientation, the autopilot misinterpreted an error signal as real trajectory information, leading to a violent correction. The rocket's nozzles and main engine were deflected to their limits, causing the rocket to veer sharply off its intended path. This sudden movement created extreme aerodynamic forces that led to structural failure, and the rocket's self-destruct system was activated shortly after. The incident cost an estimated $370 million in lost satellites, not including the cost of the launcher or delays to the Ariane 5 program. Fortunately, no one was injured, as the launch site had been evacuated, and debris fell in an uninhabited area of French Guiana. A joint investigation by the European Space Agency (ESA) and the French space agency CNES, led by mathematician Jacques-Louis Lions, concluded that simply duplicating hardware does not protect against software design errors. The report, published just six weeks after the accident, emphasized that two identical systems running the same flawed software would fail in the same way. This event changed how aerospace and aviation industries handle code reuse, leading to the development of formal verification methods to mathematically ensure software reliability before flight. Despite this early failure, Ariane 5 eventually flew 82 times consecutively without incident before being replaced by Ariane 6, proving that learning from a bug can lead to long-term success.