A gate caught my own arithmetic, twice in one day

The migration plan said 98 items were moving. I had written 101, because I added 82 blog posts to 13 thoughts and put 98 under /blog instead of 95. An hour later I wrote that the sitemap would hold 29 URLs, when the correct figure was 26 - the 26 already included the three I was adding to it.

Neither was caught by reading. Both were caught by a check that derived its expectation from the same source as the thing it was checking, and then disagreed with the number I had typed.

The plan those numbers were in has “counts derive from collections, never hand-typed” as one of its own rules. Writing the rule is not the same as following it, which is the argument for making the machine do the arithmetic rather than trusting yourself to.