Just been beaten by a bug in a production system... hidden in code silently for more than 10 years !

It just mean that for 10 years, this codepath has not been taken (The conditions for this specific error case was not met for 10 years) :-(

Actually, it would be a good monitoring information to know which path are "hot" (almost always taken since the beginning), "warm" (from time to time) or "cold" (never executed). It could help build a targetd trust. I guess that it might be possible for VM languages (like based on JVM) because the VM could monitor this... but it might be harder for machine code