How do I force homebrew to update installed software?
I don't know why this happens with Homebrew when you don't do anything to mess with it, but here's how I fixed it:


Change to your Homebrew cellar directory cd `brew --cellar`
Reset git repository git reset --hard HEAD
Upgrade pianobar, at last! brew upgrade pianobar


FYI, brew update will update Homebrew to the latest version, so you may want to do that before step 3.