Is Rails Logger truly not Thread Safe? http://log.kares.org/2011/04/railslogger-is-not-threadsafe.html
This seems like an issue that should have been picked up quite a long time ago. I just set config.threadsafe! in my config/application.rb and my log output from rails server has completely disappeared.
It looks like someone tried to fix that around a year ago - https://github.com/rails/rails/commit/d565fda5f27abf0097adf7b3e232a52f7ae5c1e9#activesupport/lib/active_support/buffered_logger.rb
What version of rails are you using ?