Written as a joke, but in my line of work (embedded systems) this is a big problem. Because most of a product's operation may depend on external inputs, it can be impossible to write a suite of unit tests that are useful without also building and automating hardware to generate the external stimuli.

We've come up with some creative responses to this, but nothing remotely like what's possible when you're just moving data around and not, e.g., testing the hardware's ability to start a 300hp diesel engine at it's low-temperature limit.