fugitive.vim gives you a nifty helper: :Gread that allows you to checkout to the previous version of the file.
Supposed that you committed some changes, but after a while you decided to get rid of those changes. As % represent the current file in fugitive.vim, to get back to the previous version, you only have to run:

:Gread head~1:%

Actually you can do something similar in any terminal window:

git checkout head~42 path/to/the/file.rb

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