Happens all the time to me. When I mis-commit a_file.txt the easiest way to revert the change from the last commit can be tricky:

git reset --soft HEAD^
git reset HEAD a_file.txt
git commit

But I figured out better way today:

git checkout HEAD^ a_file.txt
git commit -a --amend

That’s exactly one command saved plus I don’t need to dig the commit message. I know it is possible to reference it via ORIG_HEAD but I never use that and I don’t remember.