diff --git a/builtin/sparse-checkout.c b/builtin/sparse-checkout.c
index 601cf9dc278d18..61bc430f761adb 100644
--- a/builtin/sparse-checkout.c
+++ b/builtin/sparse-checkout.c
@@ -428,14 +428,26 @@ static int sparse_checkout_init(int argc, const char **argv)
 	} else
 		mode = MODE_ALL_PATTERNS;
 
-	if (set_config(mode))
-		return 1;
-
 	memset(&pl, 0, sizeof(pl));
+	pl.use_cone_patterns = core_sparse_checkout_cone;
 
 	sparse_filename = get_sparse_checkout_filename();
 	res = add_patterns_from_file_to_list(sparse_filename, "", 0, &pl, NULL, 0);
 
+	/*
+	 * If res >= 0, file already exists. If in cone mode init, verify that the
+	 * patterns are cone mode-compatible (if applicable). Otherwise, fall back
+	 * on non-cone mode sparse checkout.
+	 */
+	if (res >= 0 && core_sparse_checkout_cone && !pl.use_cone_patterns) {
+		warning(_("unable to initialize from existing patterns; disabling cone mode"));
+		mode = MODE_ALL_PATTERNS;
+		core_sparse_checkout_cone = 0;
+	}
+
+	if (set_config(mode))
+		return 1;
+
 	if (init_opts.sparse_index >= 0) {
 		if (set_sparse_index_config(the_repository, init_opts.sparse_index) < 0)
 			die(_("failed to modify sparse-index config"));
@@ -601,13 +613,21 @@ static void add_patterns_cone_mode(int argc, const char **argv,
 	add_patterns_from_input(pl, argc, argv);
 
 	memset(&existing, 0, sizeof(existing));
-	existing.use_cone_patterns = core_sparse_checkout_cone;
+	existing.use_cone_patterns = 1;
 
 	if (add_patterns_from_file_to_list(sparse_filename, "", 0,
 					   &existing, NULL, 0))
 		die(_("unable to load existing sparse-checkout patterns"));
 	free(sparse_filename);
 
+	/*
+	 * If use_cone_patterns has been disabled, at least one of the existing
+	 * patterns is invalid for cone mode. In that case, the hashmap does not
+	 * correctly reflect patterns, so we must exit early.
+	 */
+	if (existing.use_cone_patterns == 0)
+		die(_("unable to use existing sparse-checkout patterns in cone mode"));
+
 	hashmap_for_each_entry(&existing.recursive_hashmap, &iter, pe, ent) {
 		if (!hashmap_contains_parent(&pl->recursive_hashmap,
 					pe->pattern, &buffer) ||
diff --git a/dir.c b/dir.c
index ed3b06da64eed9..fffefd669c9153 100644
--- a/dir.c
+++ b/dir.c
@@ -715,6 +715,11 @@ static void add_pattern_to_hashsets(struct pattern_list *pl, struct path_pattern
 	if (!pl->use_cone_patterns)
 		return;
 
+	if (*given->pattern != '/') {
+		warning(_("unrecognized pattern: '%s'"), given->pattern);
+		goto clear_hashmaps;
+	}
+
 	if (given->flags & PATTERN_FLAG_NEGATIVE &&
 	    given->flags & PATTERN_FLAG_MUSTBEDIR &&
 	    !strcmp(given->pattern, "/*")) {
diff --git a/t/t1091-sparse-checkout-builtin.sh b/t/t1091-sparse-checkout-builtin.sh
index 8f54e69502e2be..2789b21c18d385 100755
--- a/t/t1091-sparse-checkout-builtin.sh
+++ b/t/t1091-sparse-checkout-builtin.sh
@@ -191,7 +191,7 @@ test_expect_success 'cone mode: match patterns' '
 test_expect_success 'cone mode: warn on bad pattern' '
 	test_when_finished mv sparse-checkout repo/.git/info/ &&
 	cp repo/.git/info/sparse-checkout . &&
-	echo "!/deep/deeper/*" >>repo/.git/info/sparse-checkout &&
+	echo "!/deep/deeper/*" >repo/.git/info/sparse-checkout &&
 	git -C repo read-tree -mu HEAD 2>err &&
 	test_i18ngrep "unrecognized negative pattern" err
 '
@@ -573,7 +573,7 @@ test_expect_success 'pattern-checks: starting "*"' '
 	cat >repo/.git/info/sparse-checkout <<-\EOF &&
 	/*
 	!/*/
-	*eep/
+	/*eep/
 	EOF
 	check_read_tree_errors repo "a deep" "disabling cone pattern matching"
 '
@@ -584,12 +584,21 @@ test_expect_success 'pattern-checks: contained glob characters' '
 		cat >repo/.git/info/sparse-checkout <<-EOF &&
 		/*
 		!/*/
-		something$c-else/
+		/something$c-else/
 		EOF
 		check_read_tree_errors repo "a" "disabling cone pattern matching"
 	done
 '
 
+test_expect_success 'pattern-checks: starting "/"' '
+	cat >repo/.git/info/sparse-checkout <<-\EOF &&
+	/*
+	!/*/
+	deep/
+	EOF
+	check_read_tree_errors repo "a deep" "disabling cone pattern matching"
+'
+
 test_expect_success BSLASHPSPEC 'pattern-checks: escaped characters' '
 	git clone repo escaped &&
 	TREEOID=$(git -C escaped rev-parse HEAD:folder1) &&
@@ -712,4 +721,61 @@ test_expect_success 'cone mode clears ignored subdirectories' '
 	test_cmp expect out
 '
 
+test_expect_success 'init with cone mode verifies existing cone patterns' '
+	rm -f repo/.git/info/sparse-checkout &&
+
+	# Set non-cone mode pattern
+	git -C repo sparse-checkout init &&
+	git -C repo sparse-checkout set deep/deeper*/ &&
+	git -C repo sparse-checkout disable &&
+
+	git -C repo sparse-checkout init --cone 2>err &&
+	test_i18ngrep "disabling cone mode" err &&
+	test_must_fail git -C repo config core.sparsecheckoutcone
+'
+
+test_expect_success 'add with cone mode verifies existing cone patterns' '
+	rm -f repo/.git/info/sparse-checkout &&
+
+	git -C repo sparse-checkout init --cone &&
+	cat >repo/.git/info/sparse-checkout <<-\EOF &&
+	/*
+	!/*/
+	/deep/deeper*/
+	EOF
+
+	test_must_fail git -C repo sparse-checkout add folder1 2>err &&
+	test_i18ngrep "unable to use existing sparse-checkout patterns in cone mode" err
+'
+
+# NEEDSWORK: in the case of directory patterns like `deep/`, it might be worth trying
+# to "correct" the patterns to match a cone mode style. However, that may be more difficult
+# for nested directories (like `deep/deeper1/`) in which multiple individual patterns
+# would be mapped from the original (`/deep/`, `!/deep/*/`, `/deep/deeper1/`).
+test_expect_success 'add cone pattern disallowed with existing non-cone directory pattern' '
+	rm -f repo/.git/info/sparse-checkout &&
+
+	git -C repo sparse-checkout init --cone &&
+
+	# Manually set the sparse checkout pattern to a directory pattern
+	# without preceding slash
+	cat >repo/.git/info/sparse-checkout <<-\EOF &&
+	deep/
+	EOF
+
+	# `add` fails because `deep/` is not a valid cone pattern.
+	test_must_fail git -C repo sparse-checkout add folder1/ 2>err &&
+	test_i18ngrep "unable to use existing sparse-checkout patterns in cone mode" err &&
+
+	# `set` succeeds with same patterns set properly for cone mode.
+	git -C repo sparse-checkout set deep/ folder1/ &&
+	cat >expect <<-\EOF &&
+	/*
+	!/*/
+	/deep/
+	/folder1/
+	EOF
+	test_cmp expect repo/.git/info/sparse-checkout
+'
+
 test_done