Could not load SystemTimer gem, falling back to Ruby’s slower/unsafe timeout library: no such file to load — system_timer

You have the error showing up after starting either the server (./script/server) or the console (./script/console).
For my particular case it started appearing after installing the Rails 3.0 beta3 version, and in an application developed with Rails 2.3.2.

Just install the system_timer gem:
sudo gem install system_timer