[tsavo]

This actually makes the most sense, and would help explain how the error didn't occur during testing (in good faith, I assume it was tested).

In testing, the dev may have worked from their primary to deploy the update to a series of secondary drives, then sequential performed a test boot from each secondary drive configured for each supported OS version. A shortcut/quick way to test that would've bypassed how their product updates in customer environments, also bypassing checks their software may have performed (in this case, overwriting their own file's contents).