Yes, but I might not care. If I ask for a function that takes a particular data structure and writes it to a particular file format, as long as it's not in my hot loop, I don't care how badly it's implemented (assuming it works on all my edge cases). On the other hand I might want to make sure it's maintainable or fast enough, in which case I might just write it myself.
But then you better hope that your tests are near perfect and covers every single edge case you can think of (and most of those you can't). For any given collection of tests it is quite trivial to write code which passes those tests but fails on all other input.
