When creating a collection from the sidebar of an entity page (such as https://musicbrainz.org/release-group/4159a979-cec4-366d-ad22-70df040c6940) the right entity type gets pre-selected, but other types are not blocked. As such, you can pick, say, "Area", and it will create a collection of areas, but still add the release group to it. This will then display as the collection having one entity in it (MBS-10857) that can't be displayed nor removed.