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 wo...