Bug? ignored files overwritten by checkout

[Damien Robert <damien.olivier.robert+gmane@gmail.com>]

git init
git commit --allow-empty -m "init"
git checkout -b test
echo foo > foo
git add foo
git commit -am 'add foo'
git checkout master
echo 'Important data' > foo #[1]
echo foo > .gitignore
git checkout test

If I tried a `git checkout test` after [1], I would get the error message 
    error: The following untracked working tree files would be overwritten by checkout: foo
But after adding foo to .gitignore, I am able to checkout to branch test
without warning. Of course this overwrites foo to the version in test.

I tested this in version 1.8.3.4 and 1.7.10.4.