I experienced a scenario where I was testing an install package (which was corrupt) and it brought to light a couple of PHP notices in the JInstallerHelper class.

The File:

libraries/cms/installer/helper.php line 197 & 221

Issue 1 (line 197):

$dirList = array_merge(JFolder::files($extractdir, ''), JFolder::folders($extractdir, ''));

As JFolder::files() & JFolder::folders() can return false which would break array_merge(). One solution would be type casting:

$dirList = array_merge((array)JFolder::files($extractdir, ''), (array)JFolder::folders($extractdir, ''));

Issue 2 (line 221):

if (!count($files))

Again JFolder::files() could return false so the count() of false would be 1 as opposed to 0. (see: http://php.net/manual/en/function.count.php#example-5167) Therefore as this is a false positive may I suggest something like:

if (!$files || !count($files))

or

if (!is_array($files) || !count($files))

If everyone likes the suggestions I will happily create a Pull Request.