This happened at https://www.codecademy.com/courses/learn-rails/lessons/one-many/exercises/one-many-adding-data?action=lesson_resume excercise 1/9.
Then at 5/9 I had to enter code in app/views/tags in index.html.erb and show.html.erb. I opened index file and the code was colored and I entered the text. Then I opened the show file, but the text was grey and the code I entered was not accepted: an error message telling that show.html.erb could not be found. I reset the project and logged out and in several times and then, all of a sudden, there where two files show.html.erb in the same directory. I opened them both and one had only grey text as before and the other the usual colored text. I deleted the one with the grey text and then the file was accepted.
I don't know what is happening, but sometimes the system doesn't seem very stable. I often get weird behaviour and then, 10 minutes later, everything works again.