I'm running an old #Django app that I could only upgrade to 1.10 because I needed to also upgrade Postgres>9.
That required to find precise versions of libraries that were compatible to Django 1.10 at that point in time.
New DB server gets shiny new Postgres, so I install elasticsearch on it. ES5.x.
Embarrassed to say what was my old ES version... but it was below 1. Of course, that was a problem that worked.
I use Django-haystack for faceted search on that site, and turns out that versions compatible with that Django version don't have a backend for ES2.x or higher. I had been stubbornly trying multiple 2.x versions, and turns out I had to downgrade to 1.7.6. So I finally get facets working again on my dev env.
If it was all so easy, on the server turned out I had Java 11 and ES1.x won't run with such advanced software, so also needed to downgrade to Java 8.