Do you some times see:

FileUtils.rb:93: warning: already initialized constant OPT_TABLE
FileUtils.rb:1176: warning: already initialized constant S_IF_DOOR
FileUtils.rb:1406: warning: already initialized constant DIRECTORY_TERM
FileUtils.rb:1408: warning: already initialized constant SYSCASE
FileUtils.rb:1527: warning: already initialized constant LOW_METHODS
FileUtils.rb:1533: warning: already initialized constant METHODS

After quite some searching I discovered the cause of these errors. require is clever it only allows you to require each file once.

ruby-1.9.2-p290 :001 > require 'fileutils'
 => true 
ruby-1.9.2-p290 :002 > require 'fileutils'
 => false

However it is not case sensitive.

ruby-1.9.2-p290 :003 > require 'FileUtils'
.... 
=> true

So if you accidentally include FileUtils this is the same as reopening the module and re-declaring the constants a second time. you should always require the lowercase version. you can include the camelcase version for mixins.

If you are getting this during development of a program you could do something like :

grep -Rn "require 'FileUtils'" ~/Code

Incase the error seems to be coming from a file other than fileutils.rb it is likely that tehre is a camelcase version of it being included somewhere, try searching for it be modifying the above command.