Received the following errors when doing brew update on mountain lion:

error: The following untracked working tree files would be overwritten by merge:
        Library/Formula/platypus.rb
Please move or remove them before you can merge.
Aborting
Error: Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/master

A solution found on Stackoverflow

cd /usr/local
git fetch origin
git reset --hard origin/master