The function you wrote last month is a third-party API

There is a habit I have for other people's libraries that I do not have for my own code: before I call something, I read what it returns. With my own functions I skip that, because I wrote them, so I know. Three times in three days that turned out to be false, and the third time I caught it before it cost anything only because I had started treating my own modules like somebody else's. The version I had already been burned by twice I maintain qbofile , a set of browser-based converters between the file formats accounting software uses. It is a small codebase: a parser per input format, a generator per output format, and pages that wire one to the other. Wiring a new pair felt like plumbing, so I estimated it like plumbing. Two new pages, both reusing an existing parser and an existing generator: no new code. I said that out loud before opening either end. The generator had no column for the thing the parser produced. The parser could read the category a user had assigned to each transaction; the CSV generator emitted six fixed columns and category was not one of them. Not a bug — it had simply never needed one, because the format it was originally written for does not carry categories. That is a strange kind of wrong. Nothing was broken. The code did exactly what it always had. My model of it was built from the function name. The same evening, in the same pair of modules, the second one: L . push ( `P ${ sanitizeText ( tx . description )} ` ); P is the payee field in that output format. M is the memo. Two fields, and upstream, description was defined as memo || payee . So for any transaction that had a memo, the memo took the payee slot and the actual payee was dropped. Silently — the file is valid, it imports fine, and the missing name never announces itself. The two minutes that caught the third one After the second one I wrote down a rule and did not really believe I needed it: before wiring two components together, open both ends and read what actually crosses.