The basic steps I took...

Create an user account, "someaccount"

Create a private repository named "alpha" on "someaccount"

Create a new user account, "anotheraccount"

Create a private repository named "alpha" on "anotheraccount"

Add "someaccount" as a writer to "alpha" on "anotheraccount"

Delete the repository named "alpha" on "someaccount"

From "someaccount", visit "alpha" on "anotheraccount"

Click the 'Follow' heart icon link near the top of the page for that "alpha" repository.

Click on Dashboard (signed in on "someaccount")

At this point, in the 'Following' section on the dashboard for "someaccount", I can see a repository that I am following. It is listing and pointing to my deleted repository: "alpha" on "someaccount", which no longer exists.

I believe this is a bug, for I am clicking 'Follow' for the "alpha" repository on "anotheraccount". I would expect their repository to be displayed in the 'Following' section, rather than the repository I deleted.