You will never have resources to "test absolutely everything". It is ALWAYS about building out personas and deriving tests from those.

What this tells us is that one of two things happened:

1. This was not tested at all
2. The testing harness resets the environment after every check (e.g. "does process close when killed") rather than involving a manual reset (i.e. "close and re-open task manager")

The latter is a lot more common than you would think since it makes it much easier to automate these harnesses rather than having a human at a VM. But... this is what happens when you don't step through the entire workflow.