diff --git a/Lib/enum.py b/Lib/enum.py
index 5263e51..bf3460c 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -1,5 +1,7 @@
 import sys
 from types import MappingProxyType, DynamicClassAttribute
+from operator import or_ as _or_
+from functools import reduce
 from builtins import property as _bltin_property, bin as _bltin_bin
 
 
@@ -97,6 +99,9 @@ def _iter_bits_lsb(num):
         yield b
         num ^= b
 
+def show_flag_values(value):
+    return list(_iter_bits_lsb(value))
+
 def bin(num, max_bits=None):
     """
     Like built-in bin(), except negative values are represented in
@@ -1601,14 +1606,16 @@ def __call__(self, enumeration):
                 else:
                     raise Exception('verify: unknown type %r' % enum_type)
                 if missing:
-                    raise ValueError('invalid %s %r: missing values %s' % (
+                    raise ValueError(('invalid %s %r: missing values %s' % (
                             enum_type, cls_name, ', '.join((str(m) for m in missing)))
-                            )
+                            )[:256])
+                            # limit max length to protect against DOS attacks
             elif check is NAMED_FLAGS:
                 # examine each alias and check for unnamed flags
                 member_names = enumeration._member_names_
                 member_values = [m.value for m in enumeration]
-                missing = []
+                missing_names = []
+                missing_value = 0
                 for name, alias in enumeration._member_map_.items():
                     if name in member_names:
                         # not an alias
@@ -1616,16 +1623,22 @@ def __call__(self, enumeration):
                     values = list(_iter_bits_lsb(alias.value))
                     missed = [v for v in values if v not in member_values]
                     if missed:
-                        plural = ('', 's')[len(missed) > 1]
-                        a =  ('a ', '')[len(missed) > 1]
-                        missing.append('%r is missing %snamed flag%s for value%s %s' % (
-                                name, a, plural, plural,
-                                ', '.join(str(v) for v in missed)
-                                ))
-                if missing:
+                        missing_names.append(name)
+                        missing_value |= reduce(_or_, missed)
+                if missing_names:
+                    if len(missing_names) == 1:
+                        alias = 'alias %s is missing' % missing_names[0]
+                    else:
+                        alias = 'aliases %s and %s are missing' % (
+                                ', '.join(missing_names[:-1]), missing_names[-1]
+                                )
+                    if _is_single_bit(missing_value):
+                        value = 'value 0x%x' % missing_value
+                    else:
+                        value = 'combined values of 0x%x' % missing_value
                     raise ValueError(
-                            'invalid Flag %r: %s'
-                            % (cls_name, '; '.join(missing))
+                            'invalid Flag %r: %s %s [use `enum.show_flag_values(value)` for details]'
+                            % (cls_name, alias, value)
                             )
         return enumeration
 
diff --git a/Lib/test/test_enum.py b/Lib/test/test_enum.py
index 9a7882b..ceb0da8 100644
--- a/Lib/test/test_enum.py
+++ b/Lib/test/test_enum.py
@@ -2158,9 +2158,6 @@ def __init__(self, *args):
                 self._valid = True
             @classmethod
             def _missing_(cls, value):
-                # encountered an unknown value!
-                # Luckily I'm a LenientStrEnum, so I won't crash just yet.
-                # You might want to add a new case though.
                 unknown = cls._member_type_.__new__(cls, value)
                 unknown._valid = False
                 unknown._name_ = value.upper()
@@ -3586,7 +3583,7 @@ class Bizarre(Flag):
         self.assertEqual(Bizarre.d.value, 6)
         with self.assertRaisesRegex(
                 ValueError,
-                "invalid Flag 'Bizarre': 'b' is missing named flags for values 1, 2; 'd' is missing a named flag for value 2",
+                "invalid Flag 'Bizarre': aliases b and d are missing combined values of 0x3 .use `enum.show_flag_values.value.` for details.",
             ):
             @verify(NAMED_FLAGS)
             class Bizarre(Flag):
@@ -3594,6 +3591,7 @@ class Bizarre(Flag):
                 c = 4
                 d = 6
         #
+        self.assertEqual(enum.show_flag_values(3), [1, 2])
         class Bizarre(IntFlag):
             b = 3
             c = 4
@@ -3604,13 +3602,13 @@ class Bizarre(IntFlag):
         self.assertEqual(Bizarre.d.value, 6)
         with self.assertRaisesRegex(
                 ValueError,
-                "invalid Flag 'Bizarre': 'b' is missing named flags for values 1, 2; 'd' is missing a named flag for value 2",
+                "invalid Flag 'Bizarre': alias d is missing value 0x2 .use `enum.show_flag_values.value.` for details.",
             ):
             @verify(NAMED_FLAGS)
             class Bizarre(IntFlag):
-                b = 3
                 c = 4
                 d = 6
+        self.assertEqual(enum.show_flag_values(2), [2])
 
     def test_unique_clean(self):
         @verify(UNIQUE)
@@ -3877,7 +3875,7 @@ class Missing:
 
 class MiscTestCase(unittest.TestCase):
     def test__all__(self):
-        support.check__all__(self, enum, not_exported={'bin'})
+        support.check__all__(self, enum, not_exported={'bin', 'show_flag_values'})
 
 
 # These are unordered here on purpose to ensure that declaration order