My Tests Agreed With My Code. Neither of Them Checked Reality

I had twenty-two passing tests and two separate reviewers on a piece of code. None of it objected. Then I pointed it at a real API owned by somebody else and it broke on the first live read. The mismatch fit in one sentence: my parser required ISO 8601, the documented API returned Unix seconds. The repair was not one line. It touched five files, 74 lines of parser and 52 lines of tests. The assumption was small; making it safe was not. Here is why nothing caught it, and it is the part worth keeping: My tests used ISO because my code used ISO, so they agreed with each other and never checked reality. The fixtures were written by the person who wrote the parser. They encoded the same assumption. The suite confirmed internal behaviour without ever challenging the ISO assumption, because both halves of it came from one head. Internally consistent is not the same claim as right, and nothing in that suite could tell the difference. Two separate reviewers missed it too. I cannot prove why, and I am not going to invent a reason. What I can show is that the parser and every fixture encoded the same ISO assumption, so none of the artifacts in front of anyone supplied the live contract that contradicted it. The second one was worse Working against a real system made redirect containment matter, so an independent breaker went at it. In Python 3.13 the default redirect handler rebuilds the redirected request from req.headers , dropping only content length and type. My X-API-Key sat in that header set, so the redirected request inherited it. Python has Request.add_unredirected_header() for exactly this, which marks a header as one that will not be added to a redirected request. I was not using it. The breaker reproduced it offline with a sentinel value and a cross-origin Location , and the sentinel crossed. No live FIPSign credential was ever shown to have crossed an origin. The defect was real and unshipped. I did not find it by auditing my own code, and I did not find it myself