git: prune to remove old remote tracking branches

I was trying to delete "somebranch" from a git repository today but got this error message, which baffled me for a bit:

$ git push origin :somebranch
error: unable to push to unqualified destination: somebranch
The destination refspec neither matches an existing ref on the remote nor
begins with refs/, and we are unable to guess a prefix based on the source ref.
fatal: The remote end hung up unexpectedly
error: failed to push some refs to 'git@my_server:my_repo.git'

Doing a 'git branch -a' showed a "origin/somebranch" remote branch. Why can't I delete it? Then I realized that the branch might have been deleted on the remote repository and I haven't updated my remote tracking branches yet. Doing a "git pull" won't remove remote tracking branches for branches that have been deleted. To do that for a remote named "origin", you'll need to use this command:

$ git remote prune origin