Have you found any "freebie" languages due to a combination ones you already know?

NOTE: This post/question isn't language-specific, but I have to name languages for the examples.

TL;DR: I accidentally discovered that, due to my (TL) Esperanto and Spanish knowledge, I understand Catalan fairly well despite not being sure I ever even heard of Catalan before last week.

I am learning Esperanto and Spanish. In the app I'm using I switch between them using a drop-down. The drop-down also contains all the other available languages. I accidentally tapped on Catalan (as it was near the top of the alphabetical list), and it added it to my languages. The app has a terrible user experience overall, and it's impossible to delete one once added (you have to do it via their website).

I looked it up found out it was related to Spanish, and read its Wikipedia article. They had a table of words in English, Spanish, Catalan, and other languages. I noticed that many of the Catalan words were much closer to the Esperanto words than modern Spanish.

So, I went through one of the basic lessons in Catalan, and found that I understood pretty much all of it (yeah -- it was beginner-level content, for sure). But still interesting. And more of it was familiar due to my Esperanto knowledge than my Spanish even though my Spanish knowledge is much better than my Esperanto.

Since I have frequently heard that Esperanto has a lot of basis in Latin, I added Latin to my list and checked it out. I understood practically nothing.

The question:

What other language pairs/groups have people stumbled upon? I guess the obvious ones are Spanish & Portuguese, which are largely mutually intelligible, and the Scandanavian languages.

But how about relatively obscure languages without tens of millions of current-day speakers?