I was trying to retrieve a list of episode groups. On the TV Episode Groups page, (the only page provided for Episode Groups), the query string is shown as:
GET /tv/episode_group/{id}.

Also, the Try it out area shows: https://api.themoviedb.org/3/tv/episode_group/{id}?api_key=<<api_key>>&language=en-US

In both cases, notice that the text is showing episode_group, singular. However, in order to make the list query work, I had to use episode_groups, plural - with the show id.

I figured it out after much searching when I found @TravisBell's post here. This page had great examples! Thanks for that, @TravisBell!

Perhaps it would be clearer if there was another page - or at least a comment - that the plural form, with the show id, is necessary for a list of groups:
/tv/{id}/episode_groups

while the singular form, with the specific group id, is required for a specific group?
/tv/episode_group/{id}