If you recently updated your gems, you may see this nasty error:

can't activate rubyforge (= 0.4.5), already activated rubyforge-1.0.0]

or similar. If you do, the fix is simple. Simply run:

sudo gem uninstall -a rubyforge

Run the command twice just to be sure it’s removed. Then run:

sudo gem install rubyforge

This will clear up the issue.