diff --git a/decode_test.go b/decode_test.go
index f3af685e..1c80cbf4 100644
--- a/decode_test.go
+++ b/decode_test.go
@@ -722,6 +722,18 @@ var unmarshalTests = []struct {
 		"a: 5.5\n",
 		&struct{ A jsonNumberT }{"5.5"},
 	},
+	{
+		`
+a:
+  b
+b:
+  ? a
+  : a`,
+		&M{"a": "b",
+			"b": M{
+				"a": "a",
+			}},
+	},
 }
 
 type M map[interface{}]interface{}
diff --git a/limit_test.go b/limit_test.go
index ba1c0800..e16dedc2 100644
--- a/limit_test.go
+++ b/limit_test.go
@@ -37,6 +37,8 @@ var limitTests = []struct {
 	{name: "10kb of maps", data: []byte(`a: &a [{a}` + strings.Repeat(`,{a}`, 10*1024/4-1) + `]`)},
 	{name: "100kb of maps", data: []byte(`a: &a [{a}` + strings.Repeat(`,{a}`, 100*1024/4-1) + `]`)},
 	{name: "1000kb of maps", data: []byte(`a: &a [{a}` + strings.Repeat(`,{a}`, 1000*1024/4-1) + `]`)},
+	{name: "1000kb of giant slice nested at max-depth", data: []byte(strings.Repeat(`[`, 10000) + `1` + strings.Repeat(`,1`, 1000*1024/2-20000-1) + strings.Repeat(`]`, 10000))},
+	{name: "1000kb of tokens nested at max-depth of non-simple-token maps", data: []byte("{a,a:\n" + strings.Repeat(" {a,a:", 10000-1) + `1` + strings.Repeat(",1", 1000*1024/2-6*10000-1) + strings.Repeat(`}`, 10000))},
 }
 
 func (s *S) TestLimits(c *C) {
@@ -82,6 +84,14 @@ func Benchmark1000KBMaps(b *testing.B) {
 	benchmark(b, "1000kb of maps")
 }
 
+func BenchmarkDeepSlice(b *testing.B) {
+	benchmark(b, "1000kb of giant slice nested at max-depth")
+}
+
+func BenchmarkDeepFlow(b *testing.B) {
+	benchmark(b, "1000kb of tokens nested at max-depth of non-simple-token maps")
+}
+
 func benchmark(b *testing.B, name string) {
 	for _, t := range limitTests {
 		if t.name != name {
diff --git a/scannerc.go b/scannerc.go
index 570b8ecd..9998d202 100644
--- a/scannerc.go
+++ b/scannerc.go
@@ -637,14 +637,7 @@ func yaml_parser_fetch_more_tokens(parser *yaml_parser_t) bool {
 			if !yaml_parser_stale_simple_keys(parser) {
 				return false
 			}
-
-			for i := range parser.simple_keys {
-				simple_key := &parser.simple_keys[i]
-				if simple_key.possible && simple_key.token_number == parser.tokens_parsed {
-					need_more_tokens = true
-					break
-				}
-			}
+			_, need_more_tokens = parser.simple_keys_possible_by_token_number[parser.tokens_parsed]
 		}
 
 		// We are finished.
@@ -841,14 +834,21 @@ func yaml_parser_fetch_next_token(parser *yaml_parser_t) bool {
 // cannot contain simple keys anymore.
 func yaml_parser_stale_simple_keys(parser *yaml_parser_t) bool {
 	// Check for a potential simple key for each flow level.
-	for i := range parser.simple_keys {
+	for i := parser.simple_keys_min_possible_index; i < len(parser.simple_keys); i++ {
 		simple_key := &parser.simple_keys[i]
 
+		if !simple_key.possible {
+			// If this key was already marked as not possible, just move on.
+			// This might be the uninitialized key at the top of the stack, so
+			// keep min_possible at i instead of i + 1.
+			parser.simple_keys_min_possible_index = i
+			continue
+		}
 		// The specification requires that a simple key
 		//
 		//  - is limited to a single line,
 		//  - is shorter than 1024 characters.
-		if simple_key.possible && (simple_key.mark.line < parser.mark.line || simple_key.mark.index+1024 < parser.mark.index) {
+		if simple_key.mark.line < parser.mark.line || simple_key.mark.index+1024 < parser.mark.index {
 
 			// Check if the potential simple key to be removed is required.
 			if simple_key.required {
@@ -857,8 +857,15 @@ func yaml_parser_stale_simple_keys(parser *yaml_parser_t) bool {
 					"could not find expected ':'")
 			}
 			simple_key.possible = false
+			delete(parser.simple_keys_possible_by_token_number, simple_key.token_number)
+			parser.simple_keys_min_possible_index = i + 1
+		} else {
+			// Once we find a key that's not stale, the rest will be on the
+			// same line and shorter, so don't need to be checked now.
+			break
 		}
 	}
+
 	return true
 }
 
@@ -885,6 +892,8 @@ func yaml_parser_save_simple_key(parser *yaml_parser_t) bool {
 		if !yaml_parser_remove_simple_key(parser) {
 			return false
 		}
+		parser.simple_keys_possible_by_token_number[simple_key.token_number] = struct{}{}
+
 		parser.simple_keys[len(parser.simple_keys)-1] = simple_key
 	}
 	return true
@@ -900,9 +909,13 @@ func yaml_parser_remove_simple_key(parser *yaml_parser_t) bool {
 				"while scanning a simple key", parser.simple_keys[i].mark,
 				"could not find expected ':'")
 		}
+		// Remove the key from the stack.
+		parser.simple_keys[i].possible = false
+		delete(parser.simple_keys_possible_by_token_number, parser.simple_keys[i].token_number)
+	}
+	if parser.simple_keys_min_possible_index > i {
+		parser.simple_keys_min_possible_index = i
 	}
-	// Remove the key from the stack.
-	parser.simple_keys[i].possible = false
 	return true
 }
 
@@ -928,7 +941,14 @@ func yaml_parser_increase_flow_level(parser *yaml_parser_t) bool {
 func yaml_parser_decrease_flow_level(parser *yaml_parser_t) bool {
 	if parser.flow_level > 0 {
 		parser.flow_level--
-		parser.simple_keys = parser.simple_keys[:len(parser.simple_keys)-1]
+		last := len(parser.simple_keys) - 1
+		if parser.simple_keys[last].possible {
+			delete(parser.simple_keys_possible_by_token_number, parser.simple_keys[last].token_number)
+		}
+		parser.simple_keys = parser.simple_keys[:last]
+		if parser.simple_keys_min_possible_index > last {
+			parser.simple_keys_min_possible_index = last
+		}
 	}
 	return true
 }
@@ -1005,6 +1025,8 @@ func yaml_parser_fetch_stream_start(parser *yaml_parser_t) bool {
 	// Initialize the simple key stack.
 	parser.simple_keys = append(parser.simple_keys, yaml_simple_key_t{})
 
+	parser.simple_keys_possible_by_token_number = make(map[int]struct{})
+
 	// A simple key is allowed at the beginning of the stream.
 	parser.simple_key_allowed = true
 
@@ -1304,6 +1326,7 @@ func yaml_parser_fetch_value(parser *yaml_parser_t) bool {
 
 		// Remove the simple key.
 		simple_key.possible = false
+		delete(parser.simple_keys_possible_by_token_number, simple_key.token_number)
 
 		// A simple key cannot follow another simple key.
 		parser.simple_key_allowed = false
diff --git a/yamlh.go b/yamlh.go
index e25cee56..2a6b8385 100644
--- a/yamlh.go
+++ b/yamlh.go
@@ -577,8 +577,10 @@ type yaml_parser_t struct {
 	indent  int   // The current indentation level.
 	indents []int // The indentation levels stack.
 
-	simple_key_allowed bool                // May a simple key occur at the current position?
-	simple_keys        []yaml_simple_key_t // The stack of simple keys.
+	simple_key_allowed                   bool                // May a simple key occur at the current position?
+	simple_keys                          []yaml_simple_key_t // The stack of simple keys.
+	simple_keys_min_possible_index       int                 // Where in the stack should we restart scanning for staleness?
+	simple_keys_possible_by_token_number map[int]struct{}    // Does this token_number represent a possible simple_key?
 
 	// Parser stuff