Actually, they get lots of fan mail criticising many of their tests and they regularly go back and re-test, taking the criticisms into account. Sometimes the outcomes change, other times they remain the same. I do think that they try to be as fair and honest with their testing as they can be, and admit it when they get it wrong.
Sometimes yes, sometimes it is predicated on "Can we make this thing explode"? The Robin Hood one is a famous example of how the "proved" something can't be done, yet the intro shows it (Errol Flynn as Robin Hood splitting an arrow).