Access denied issue

Often, but not always when adding plugins I will get an access denied message. I have discovered however that if I just tap the back arrow on the browser the script will continue with no problem.

What’s going on here? I had suspected that the wp-admin folder permission might be awry, but that doesn’t seem to be the case.

-rw-rw-r–+ 1 _www admin 4951 Aug 20 2014 wp-activate.php
drwxrwxr-x+ 89 _www admin 3026 Apr 3 08:04 wp-admin
-rw-rw-r–+ 1 _www admin 271 Jan 8 2012 wp-blog-header.php
-rw-rw-r–+ 1 _www admin 5008 Nov 26 13:17 wp-comments-post.php
-rw-rw-r–+ 1 _www admin 2726 Sep 8 2014 wp-config-sample.php
-rw-r–r–+ 1 _www admin 2921 Apr 3 08:04 wp-config.php
drwxrwxr-x+ 18 _www admin 612 Apr 4 09:33 wp-content
-rw-rw-r–+ 1 _www admin 2956 May 12 2014 wp-cron.php
drwxrwxr-x+ 125 _www admin 4250 Feb 18 15:10 wp-includes
-rw-rw-r–+ 1 _www admin 2380 Oct 24 2013 wp-links-opml.php
-rw-rw-r–+ 1 _www admin 2714 Mar 30 13:41 wp-load.php
-rw-rw-r–+ 1 _www admin 33435 Dec 16 15:19 wp-login.php
-rw-rw-r–+ 1 _www admin 8252 Jul 17 2014 wp-mail.php
-rw-rw-r–+ 1 _www admin 11115 Jul 18 2014 wp-settings.php
-rw-rw-r–+ 1 _www admin 25152 Nov 30 14:23 wp-signup.php
drwxrwxr-x+ 13 _www admin 442 Apr 1 12:17 wp-snapshots
-rw-rw-r–+ 1 _www admin 4035 Nov 30 14:23 wp-trackback.php
-rw-rw-r–+ 1 _www admin 3032 Feb 9 2014 xmlrpc.php