Solo dev shipping 10 UI languages: what silently broke?

Post-mortem from last week.

My site is translated into 10 languages. The language switcher and the internal links on every localised page were supposed to keep visitors in their language. For months they did not. Every locale page quietly linked back to the English homepage.

The cause was one line. A lookup used slashless keys like /for/bakeries while the site's own canonical path helper appended a slash. Every lookup missed, returned an empty array, and nothing threw or logged. 376 pages shipped like that.

The fix was one line, in two files, because the manifest generator and the build were separate steps.

Lesson I took: a function that returns empty instead of throwing will hide a bug for as long as you let it.

What is the silent bug that lived longest in your product before you noticed?