Sprockets manifest issues (when jumping between Rails 4/5)

Are you seeing errors like this?

ActionView::Template::Error:
       no implicit conversion of String into Integer

ActionView::Template::Error:
            Asset `loading.gif` was not declared to be precompiled in production.
            Declare links to your assets in `app/assets/config/manifest.js`.

Are you using Sprockets and switching between Rails 4 and Rails 5?
Is it driving you to drink?

All you need to do is
rake assets:clobber tmp:clean
and you'll be on your way!