In Heroku when you relase a new version with Rails, you have to remember the migration for the database:
git push heroku master && heroku run rails db:migrate
It’s easy to forget the second part and the application fail only after someone open a page.
With a new feature of Heroku (3 August 2016) inside Procfile you can easily fix this problem with a single line:
# Procfile release: bundle exec rails db:migrate