How many times has the stable and established X surprised you with some unexpected behavior?

Once upon a time I wanted a URL /like/this/with%2Fslashes/ where a path parameter could, in rare cases, contain an encoded slash. Legit according to every RFC I checked. Works fine in browsers. But I couldn't get the web server to not convert it to a / before hitting my code. And woe betide thee who wants to make nginx return a custom error code 444.

RFC 3986 treats %2F as a reserved character that behavior becomes app specific.