os/exec.Command returns a Cmd with only Path and Args filled out.

That means that doing

cmd := exec.Command("foo")
cmd.Env = append(cmd.Env, "FOO=bar")

is always equivalent to

cmd := exec.Command("foo")
cmd.Env = []string{"FOO=bar"}

but almost certainly intended to be

cmd := exec.Command("foo")
cmd.Env = append(cmd.Environ(), "FOO=bar")

instead.

I introduced this bug at least a couple times in the standard library tests.