> However, that same code may have been in production for decades, proving fit for purpose by example.
Until it isn't anymore because one of the critical but undocumented (and possibly even unintended) assumptions no longer applies. See also: Ariane 5. I can easily see that happening in this case - all they'd have to do is shrink the available stack space to free up RAM elsewhere and suddenly critical variables would intermittently get corrupted, possibly only under circumstances that didn't happen in testing.
Until it isn't anymore because one of the critical but undocumented (and possibly even unintended) assumptions no longer applies. See also: Ariane 5. I can easily see that happening in this case - all they'd have to do is shrink the available stack space to free up RAM elsewhere and suddenly critical variables would intermittently get corrupted, possibly only under circumstances that didn't happen in testing.