Uh, where can you use "head"?

    $ git show head
    fatal: ambiguous argument 'head': unknown revision or path not in the working tree.
    Use '--' to separate paths from revisions, like this:
    'git <command> [<revision>...] -- [<file>...]'
    $ git show head --
    fatal: bad revision 'head'
    $ git version
    git version 1.8.4.2

most likely on case-insensitive file systems, like the default HFS+ on the Mac and NTFS on Windows.

EDIT: to expand, reference lookups are filysystem lookups:

    01:42:24 (gh-pages) [bpowers@fina myproject]$ strace git show HEAD 2>&1 | grep HEAD
    execve("/usr/bin/git", ["git", "show", "HEAD"], [/* 79 vars */]) = 0
    lstat(".git/HEAD", {st_mode=S_IFREG|0664, st_size=25, ...}) = 0

EDIT2: formatting

Aaah, good point, that's probably the cause. Yeah, this is on a Mac.

It's also definitely more transparent for a new user.

git rebase @~4 looks like Perl :)