If I recall correctly, the erroring code originated from the Ariane 4, where it was required to align the vehicle after launch. It was a bit of a perfect storm: the code was left in for diagnostic purposes, a steeper trajectory in the Ariane 5 triggered the bug, and the system had supposedly been battle-tested in Ariane 4.
It's been a while but I wrote about this here: https://blog.bugsnag.com/bug-day-ariane-5-disaster/