I was looking to upgrade my Scala installation and thought I'd update Homebrew before doing so. I got the following error message when performing the update:

mac-attack:~ ewilson$ sudo brew update
Checking out files: 100% (3246/3246), done.
warning: inexact rename detection was skipped due to too many files.
warning: you may want to set your diff.renamelimit variable to at least 1019 and retry the command.
Error: undefined method `to_sym' for nil:NilClass
Please report this bug:
    https://github.com/mxcl/homebrew/wiki/troubleshooting
/usr/local/Library/Homebrew/cmd/update.rb:97:in `report'
/usr/local/Library/Homebrew/cmd/update.rb:95:in `each_line'
/usr/local/Library/Homebrew/cmd/update.rb:95:in `report'
/usr/local/Library/Homebrew/cmd/update.rb:18:in `update'
/usr/local/bin/brew:94:in `send'
/usr/local/bin/brew:94

Obviously not what I was expecting as brew is usually pretty reliable. After doing some FAQ reading I came across this command:

mac-attack:~ ewilson$ sudo brew doctor

The command provided me with some recommendations of configs, paths and symlinks that needed to be corrected. I followed the guidance and my issues were solved.