diff --git a/src/autocmd.c b/src/autocmd.c
index 341cf83..7eb08c8 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -921,16 +921,30 @@
 		if ((STRNCMP(cmd, "++nested", 8) == 0 && VIM_ISWHITE(cmd[8])))
 		{
 		    if (nested)
+		    {
 			semsg(_(e_duplicate_argument_str), "++nested");
+			return;
+		    }
 		    nested = TRUE;
 		    cmd = skipwhite(cmd + 8);
 		}
 
-		// Check for the old "nested" flag.
+		// Check for the old "nested" flag in legacy script.
 		if (STRNCMP(cmd, "nested", 6) == 0 && VIM_ISWHITE(cmd[6]))
 		{
+		    if (in_vim9script())
+		    {
+			// If there ever is a :nested command this error should
+			// be removed and "nested" accepted as the start of the
+			// command.
+			emsg(_(e_invalid_command_nested_did_you_mean_plusplus_nested));
+			return;
+		    }
 		    if (nested)
+		    {
 			semsg(_(e_duplicate_argument_str), "nested");
+			return;
+		    }
 		    nested = TRUE;
 		    cmd = skipwhite(cmd + 6);
 		}
diff --git a/src/errors.h b/src/errors.h
index 5ac7b3f..e08186e 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -2786,7 +2786,10 @@
 # endif
 EXTERN char e_missing_argument_type_for_str[]
 	INIT(= N_("E1077: Missing argument type for %s"));
-// E1078 unused
+#endif
+EXTERN char e_invalid_command_nested_did_you_mean_plusplus_nested[]
+	INIT(= N_("E1078: Invalid command \"nested\", did you mean \"++nested\"?"));
+#ifdef FEAT_EVAL
 EXTERN char e_cannot_declare_variable_on_command_line[]
 	INIT(= N_("E1079: Cannot declare a variable on the command line"));
 EXTERN char e_invalid_assignment[]
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index d3c9b89..7be0c18 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
Binary files differ
diff --git a/src/version.c b/src/version.c
index e1ca0d7..5f703c4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4505,
+/**/
     4504,
 /**/
     4503,