Please take a minute or two to update your project Gemfile and run the build on the RCs. Spotting regressions now means they're fixed in another RC rather than a new point release some month(s) away.
MySQL and Postgres Active Record adapters were enhanced to avoid confusing behaviour or exceptions when running migrations on multi-instance clusters. See the issue as well for more details.
Good old serve_static_files is being deprecated as of Rails 5 in favor of public_file_server.enabled so we have all static asset options under the same public_file_server key.
This was reported quite a while ago, but now someone stepped up to the plate and followed it through. Rails got a bit safer again, thanks to the community. ❤️