diff --git a/usr.sbin/mergemaster/mergemaster.8 b/usr.sbin/mergemaster/mergemaster.8
index a56897c31d75..27157918326b 100644
--- a/usr.sbin/mergemaster/mergemaster.8
+++ b/usr.sbin/mergemaster/mergemaster.8
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 7, 2010
+.Dd March 13, 2010
 .Dt MERGEMASTER 8
 .Os
 .Sh NAME
@@ -32,7 +32,8 @@
 .Nd merge configuration files, et al during an upgrade
 .Sh SYNOPSIS
 .Nm
-.Op Fl scrvahipFCPU
+.Op Fl scrvhpCP
+.Op Fl a|iFU
 .Op Fl m Ar /path/to/sources
 .Op Fl t Ar /path/to/temp/root
 .Op Fl d
@@ -168,7 +169,7 @@ and therefore can override both files.
 When the comparison is done if there are any files remaining
 in the temproot directory they will be listed, and if the
 .Fl a
-option is not in use the user will be given the option of 
+option is not in use the user will be given the option of
 deleting the temproot directory.
 If there are no files remaining in the temproot directory
 it will be deleted.
@@ -206,9 +207,11 @@ If the
 directory exists, it creates a new one in a previously
 non-existent directory.
 This option unsets the verbose flag,
-but other than
-.Fl U
-it is compatible with all other options.
+and is not compatible with
+.Fl i ,
+.Fl F ,
+or
+.Fl U .
 Setting
 .Fl a
 makes
diff --git a/usr.sbin/mergemaster/mergemaster.sh b/usr.sbin/mergemaster/mergemaster.sh
index 2fa99066a635..7f0696846e1b 100755
--- a/usr.sbin/mergemaster/mergemaster.sh
+++ b/usr.sbin/mergemaster/mergemaster.sh
@@ -15,7 +15,7 @@ PATH=/bin:/usr/bin:/usr/sbin
 display_usage () {
   VERSION_NUMBER=`grep "[$]FreeBSD:" $0 | cut -d ' ' -f 4`
   echo "mergemaster version ${VERSION_NUMBER}"
-  echo 'Usage: mergemaster [-scrvahipFCPU]'
+  echo 'Usage: mergemaster [-scrvhpCP] [-a|[-iFU]]'
   echo '    [-m /path] [-t /path] [-d] [-u N] [-w N] [-A arch] [-D /path]'
   echo "Options:"
   echo "  -s  Strict comparison (diff every pair of files)"
@@ -337,6 +337,18 @@ while getopts ":ascrvhipCPm:t:du:w:D:A:FU" COMMAND_LINE_ARGUMENT ; do
   esac
 done
 
+if [ -n "$AUTO_RUN" ]; then
+  if [ -n "$FREEBSD_ID" -o -n "$AUTO_UPGRADE" -o -n "$AUTO_INSTALL" ]; then
+    echo ''
+    echo "*** You have included the -a option along with one or more options"
+    echo '    that indicate that you wish mergemaster to actually make updates'
+    echo '    (-F, -U, or -i), however these options are not compatible.'
+    echo '    Please read mergemaster(8) for more information.'
+    echo ''
+    exit 1
+  fi
+fi
+
 # Assign the location of the mtree database
 #
 MTREEDB=${MTREEDB:-${DESTDIR}/var/db}