[spr93]

The (possibly apocryphal) version I was told includes another layer: The problematic code was part of a last-minute kludge for the Ariane 4. The kludged code violated the Ariane 4's specifications. The Ariane 4 team knew this, but the code "couldn't" be triggered under the Ariane 4's conditions.

A few years later, the code is pulled out for re-use in the Ariane 5. Apparently the kludge wasn't documented and no one bothered to confirm that the Ariane 4 code was actually up to the Ariane 4 spec. Had the Ariane 4 code been up to spec, the code reuse would have been fine.