A GTFS URL should answer with a zip. Some answer with a cookie banner, a login
form, a "this page has moved" notice or a JSON error, all with a cheerful
200 OK, so every client thinks the download worked until it tries to unzip
it.
The law
The feed's URL answers 200 with a body that is HTML, JSON or XML instead of a zip.
How to get pardoned
Serve the zip. If the feed has moved, redirect the old URL to the new one; if it is gone, answer 404 or 410.