Note that if you fail to specify a revision then you have to include a separator. Example from my console:
git reset <path_to_file>
fatal: ambiguous argument '<path_to_file>': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions
git reset -- <path_to_file>
Unstaged changes after reset:
M <path_to_file>
(gitGit version 1.7.5.4)