In the ftools.rb implementation of File.makedirs (ruby 1.8.1-preview3), 
there is apparently a race condition. It can result in an error like:

/usr/local/lib/ruby/1.8/ftools.rb:139:in `mkdir': File exists - 
/tmp/fsdb-test/TestConcurrency14475//Results (Errno::EEXIST)

   def makedirs(*dirs)
     verbose = if dirs[-1].is_a? String then false else dirs.pop end
#    mode = if dirs[-1].is_a? Fixnum then dirs.pop else 0755 end
     mode = 0755
     for dir in dirs
       parent = dirname(dir)
       next if parent == dir or FileTest.directory? dir
       makedirs parent unless FileTest.directory? parent
       $deferr.print "mkdir ", dir, "\n" if verbose
       if basename(dir) != ""
	Dir.mkdir dir, mode
       end
     end
   end

This can be avoided by simply rescuing that exception around the 
Dir.mkdir call:

   def makedirs(*dirs)
     verbose = if dirs[-1].is_a? String then false else dirs.pop end
#    mode = if dirs[-1].is_a? Fixnum then dirs.pop else 0755 end
     mode = 0755
     for dir in dirs
       parent = dirname(dir)
       next if parent == dir or FileTest.directory? dir
       makedirs parent unless FileTest.directory? parent
       $deferr.print "mkdir ", dir, "\n" if verbose
       if basename(dir) != ""
         begin
	  Dir.mkdir dir, mode
         rescue Errno::EEXIST
         end
       end
     end
   end

(Maybe in the verbose case this exception should be logged to $deferr.)