A download that passes every check and still fails

Three of seven releases went out broken, and every one had passed a check. A screenwriting app download can verify perfectly and still fail.

The three broke in three different ways that turned out to be one mistake. The signature verified: it was a real signature, just not one issued by anyone the buyer's Mac trusts, so it verified only against itself. The download looked present: it was, to the one account that could always see it, and to nobody else.

In each case the check asked a question that the check's own side of the boundary could answer without going outside. A green light from inside the room.

The test that would have caught all three is simple. Fetch the file the way a stranger would, and look at it with the same eyes. That sounds obvious written down, and it is the last thing anyone thinks of, because the near side is where you live.

You can revise a scene until it reads clean to you and still lose the reader in the first line. The proof that counts is the one outside your own head, and for an installer that means fetching it the way a stranger would.