it turned out the issue had been there - latent - from day one. But it took a change to an interface to bring it to the surface.
Not a computer person, but have been tinkering with computers since the early 80's, and have some good friends with PhD's in the area. That is exactly the sort of bug that concerns pilots with only a dual FCC system moving a very large control surface. The latent one that may surface much much much later and catch someone out.
Much earlier in the I posted a link to one of the original forensic computer accident investigations. it too involved this exact type of latent bug sitting silently waiting to reveal itself with an "interface change" (removal of a physical mechanical interlock preventing a lethal dose of radiation):
Nancy Leveson: Therac-25 Accident. This case wan't a race condition, rather a simple "out by one" coding error that remained dormant for years.