病情：

error@cox:/Users/mom$ brew update
remote: Counting objects: 2482, done.
remote: Compressing objects: 100% (750/750), done.
remote: Total 2105 (delta 1543), reused 1822 (delta 1349)
Receiving objects: 100% (2105/2105), 329.33 KiB | 122 KiB/s, done.
Resolving deltas: 100% (1543/1543), completed with 295 local objects.
From https://github.com/mxcl/homebrew
   f884112..28f7e01  master     -> origin/master
error: The following untracked working tree files would be overwritten by merge:
	Library/Formula/autojump.rb
Please move or remove them before you can merge.
Updating f884112..28f7e01
Aborting
Error: Failed while executing git pull  origin refs/heads/master:refs/remotes/origin/master

解决方法：

cd `brew --prefix`

git clean -n # Does a dry run---shows what will get nuked without touching anything
git clean -f # This will destroy files