Ariane 5’s “Reused Code” Catastrophe

On June 4, 1996, Ariane 5 Flight 501 exploded 37 seconds after launch due to a software failure rooted in reused code from the Ariane 4 rocket. An alignment routine inherited from Ariane 4 continued running during Ariane 5's ascent despite serving no purpose there. Ariane 5's higher horizontal velocity caused a 64-bit float-to-16-bit integer conversion to overflow — a conversion that lacked exception handling because engineers assumed the value could never exceed safe bounds. Both redundant inertial reference units failed, and the flight computer misinterpreted diagnostic data as valid guidance, commanding fatal steering corrections. The deeper lesson is organizational: reusing code also reuses its embedded assumptions, which become invisible liabilities when the operating context changes. Teams inheriting systems must actively revalidate assumptions about scale, performance envelopes, and failure modes rather than trusting that 'it worked before' means 'it is safe now.'