I have not done Ruby but I found a lot of my <100% stuff to be weird and unlikely errors - logic error "I violated an invariant somewhere else" type of thing, or syscalls / standard library functions that could error out though one would never expect it. So say I `open()` a file and then immediately `fstat()` it- I don't ever expect the `fstat()` to fail, I'll write the check, log, and failure/crash just in case it does come up, but rigging up a successful open() / failed fstat() for the test seems a bit much.