This happens for example on https://musicbrainz.org/ws/2/release-group?query=arid:4e008872-ec16-4a61-8369-efb0c14901fa&limit=100 with a few reloads (
The client (i.e. browser) should not receive a 400 response because RFC 2616 says about the 400 status code that
The request could not be understood by the server due to malformed
syntax. The client SHOULD NOT repeat the request without
Simply reloading can result in a "proper" response on the URL above so the client did nothing wrong.