Or insufficient testing?

Testing isn't an effective way to catch most race conditions. Code reviews, static analysis, and rigorous enforcement of concurrent coding standards is usually a better approach.

Or maybe even just reviewing logic that is subject to pre-emption.

Maybe I'm being too harsh.. on the plus side the system has at least failed hard every time there's been a fault. Nobody has died. But it's been 3 times now in the past couple of years, and two of those times resulted in over 2000 flights cancelled and days of backlog, and misery for hundreds of thousands. It's really not acceptable.