Index: diff.c
===================================================================
--- diff.c	(revision 357993)
+++ diff.c	(working copy)
@@ -100,6 +100,7 @@
 };
 
 void usage(void) __dead2;
+void conflicting_format(void) __dead2;
 void push_excludes(char *);
 void push_ignore_pats(char *);
 void read_excludes_file(char *file);
@@ -141,6 +142,8 @@
 			break;
 		case 'C':
 		case 'c':
+			if (diff_format != D_NORMAL)
+				conflicting_format();
 			cflag = 1;
 			diff_format = D_CONTEXT;
 			if (optarg != NULL) {
@@ -154,13 +157,19 @@
 			dflags |= D_MINIMAL;
 			break;
 		case 'D':
+			if (diff_format != D_NORMAL)
+				conflicting_format();
 			diff_format = D_IFDEF;
 			ifdefname = optarg;
 			break;
 		case 'e':
+			if (diff_format != D_NORMAL)
+				conflicting_format();
 			diff_format = D_EDIT;
 			break;
 		case 'f':
+			if (diff_format != D_NORMAL)
+				conflicting_format();
 			diff_format = D_REVERSE;
 			break;
 		case 'H':
@@ -193,10 +202,12 @@
 			Nflag = 1;
 			break;
 		case 'n':
+			if (diff_format != D_NORMAL)
+				conflicting_format();
 			diff_format = D_NREVERSE;
 			break;
 		case 'p':
-			if (diff_format == 0)
+			if (diff_format == D_NORMAL)
 				diff_format = D_CONTEXT;
 			dflags |= D_PROTOTYPE;
 			break;
@@ -207,6 +218,8 @@
 			rflag = 1;
 			break;
 		case 'q':
+			if (diff_format != D_NORMAL)
+				conflicting_format();
 			diff_format = D_BRIEF;
 			break;
 		case 'S':
@@ -223,6 +236,8 @@
 			break;
 		case 'U':
 		case 'u':
+			if (diff_format != D_NORMAL)
+				conflicting_format();
 			diff_format = D_UNIFIED;
 			if (optarg != NULL) {
 				l = strtol(optarg, &ep, 10);
@@ -249,9 +264,13 @@
 			push_excludes(optarg);
 			break;
 		case 'y':
+			if (diff_format != D_NORMAL)
+				conflicting_format();
 			diff_format = D_SIDEBYSIDE;
 			break;
 		case OPT_CHANGED_GROUP_FORMAT:
+			if (diff_format != D_NORMAL)
+				conflicting_format();
 			diff_format = D_GFORMAT;
 			group_format = optarg;
 			break;
@@ -491,3 +510,10 @@
 
 	exit(2);
 }
+
+void
+conflicting_format(void)
+{
+	fprintf(stderr, "error: conflicting output format options.\n");
+	usage();
+}
Index: tests/diff_test.sh
===================================================================
--- tests/diff_test.sh	(revision 357993)
+++ tests/diff_test.sh	(working copy)
@@ -11,6 +11,7 @@
 atf_test_case b230049
 atf_test_case Bflag
 atf_test_case tabsize
+atf_test_case conflicting_format
 
 simple_body()
 {
@@ -49,8 +50,6 @@
 {
 	atf_check -o file:$(atf_get_srcdir)/unified_p.out -s eq:1 \
 		diff -up -L input_c1.in -L input_c2.in  "$(atf_get_srcdir)/input_c1.in" "$(atf_get_srcdir)/input_c2.in"
-	atf_check -o file:$(atf_get_srcdir)/unified_c9999.out -s eq:1 \
-		diff -u -c9999 -L input_c1.in -L input_c2.in "$(atf_get_srcdir)/input_c1.in" "$(atf_get_srcdir)/input_c2.in"
 	atf_check -o file:$(atf_get_srcdir)/unified_9999.out -s eq:1 \
 		diff -u9999 -L input_c1.in -L input_c2.in "$(atf_get_srcdir)/input_c1.in" "$(atf_get_srcdir)/input_c2.in"
 }
@@ -175,6 +174,18 @@
 	    diff -t --tabsize 1 A B
 }
 
+conflicting_format_body()
+{
+	printf "\tA\n" > A
+	printf "\tB\n" > B
+	
+	atf_check -s exit:2 -e ignore diff -c -u A B 
+	atf_check -s exit:2 -e ignore diff -e -f A B 
+	atf_check -s exit:2 -e ignore diff -y -q A B 
+	atf_check -s exit:2 -e ignore diff -q -u A B 
+	atf_check -s exit:2 -e ignore diff -q -c A B 
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case simple
@@ -188,4 +199,5 @@
 	atf_add_test_case b230049
 	atf_add_test_case Bflag
 	atf_add_test_case tabsize
+	atf_add_test_case conflicting_format 
 }