In the Getting Started guide there's a sentence:
"If you now go to http://localhost:3000/posts/new you'll almost be able to create a post. Try it! You should get an error that looks like this:" and image of "ActiveModel::ForbiddenAttributesError"
actually the error doesn't occur because earlier in the guide you enter this code:
This was fixed with efeb039 but as far as I can see it was not backported on 4-0-stable.
backport efeb039 from #11201 fixes #11540 [ci skip]