On StackExchange, when you want to add a community to your account, you go into the upper left corner, click on the pull down, and click "edit" next to the "Your Communities" text.
Then you start typing the name of the community you want to add, click on the community as it comes up during the suggestions, hit the add button, and presto! you've added a new community to your list...
...except you haven't. You need to scroll down to the bottom of your list, past the newly added community names (that look the same as previously added community names), and hit the save button.
Should there be a save button at all?
I would argue no, because it's hard to accidentally add a site that you didn't mean to (since only valid names can be added, which you either have to type out correctly or choose from a pull down of suggested name), and because it's easy to delete a community (just hit the 'x') but not TOO easy that you may accidentally delete one (the 'x' button is small).
Moreover, if there should be a save button, shouldn't it be next to the add button? If your list of communities is more than ~8 long, the user can't even see that a save button exists.
Although this question is about StackExchange, I would like to know what the general case usability answer would be.
Related question: Do "add" or "edit" functions need a confirm button?
However, in that question you're allowed to add any text you want to a list, whereas in the StackExchange case, you can only add communities that actually exist (ie you had to type the community name exactly correctly, or consciously pick a community from the suggested drop downs).