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!