I may be missing a step here but I'm not sure what's going on. I know that Reddit may recommend other subreddits to users but I've never heard of random posts showing up in feeds.
I mod a sub for a big city. Among the things we do is have a mechanism to restrict access, via subreddit karma, to certain posts. We explain both in the post and in removal messages why this is done and what they need to gain access to the restricted posts. Restricted posts tend to be hot topics and tend to be popular on the sub.
Today, especially tonight, we've gotten at least a dozen people in mod-mail complaining about being unable to comment on a restricted post. Most of them seem to feel that we "sent them" the post but after a while it became clear that this is randomly showing up in their feed. ALL of them have a big post history in two nearby states. (While a very few restricted posts escape orbit and hit /All or /Popular, this particular post has not escaped.)
A dozen isn't a lot but it's worrying that this might continue.
Has something changed that I've missed or is something odd going on? I took a look at the other two subs and there's nothing there that says they recommend the city sub, and I can't see why they would.
Edit: Forgot to mention -- only one of these users were new to Reddit. For them it might be that they didn't have enough subs in their feed to not get random crap, but most of the others were 5+ year old accounts who seemed to have enough activity in other subs that getting random posts seems less likely.