diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -1194,7 +1194,7 @@ class EscapeDecodeTest(unittest.TestCase):
         check(b"[\\\n]", b"[]")
         check(br'[\"]', b'["]')
         check(br"[\']", b"[']")
-        check(br"[\\]", br"[\]")
+        check(br"[\\]", b"[\\]")
         check(br"[\a]", b"[\x07]")
         check(br"[\b]", b"[\x08]")
         check(br"[\t]", b"[\x09]")
@@ -1203,7 +1203,6 @@ class EscapeDecodeTest(unittest.TestCase):
         check(br"[\f]", b"[\x0c]")
         check(br"[\r]", b"[\x0d]")
         check(br"[\7]", b"[\x07]")
-        check(br"[\8]", br"[\8]")
         check(br"[\78]", b"[\x078]")
         check(br"[\41]", b"[!]")
         check(br"[\418]", b"[!8]")
@@ -1211,12 +1210,12 @@ class EscapeDecodeTest(unittest.TestCase):
         check(br"[\1010]", b"[A0]")
         check(br"[\501]", b"[A]")
         check(br"[\x41]", b"[A]")
-        check(br"[\X41]", br"[\X41]")
         check(br"[\x410]", b"[A0]")
         for b in range(256):
             if b not in b'\n"\'\\abtnvfr01234567x':
                 b = bytes([b])
-                check(b'\\' + b, b'\\' + b)
+                with self.assertWarns(DeprecationWarning):
+                    decode(b"\\" + b)
 
     def test_errors(self):
         decode = codecs.escape_decode
diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -2729,6 +2729,10 @@ class UnicodeTest(string_tests.CommonTest,
         support.check_free_after_iterating(self, iter, str)
         support.check_free_after_iterating(self, reversed, str)
 
+    def test_invalid_sequences(self):
+        for letter in "cdeghijklmopqswyzABCDEFGHIJKLMOPQRSTVWXYZ":
+            with self.assertWarns(DeprecationWarning):
+                eval(r"'\%s'" % letter)
 
 class StringModuleTest(unittest.TestCase):
     def test_formatter_parser(self):
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -1207,8 +1207,9 @@ PyObject *PyBytes_DecodeEscape(const char *s,
             break;
 
         default:
+            if (PyErr_WarnFormat(PyExc_DeprecationWarning, 1, "invalid escape sequence '%c'", *(--s)) < 0)
+                goto failed;
             *p++ = '\\';
-            s--;
             goto non_esc; /* an arbitrary number of unescaped
                              UTF-8 bytes may follow. */
         }
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -5848,6 +5848,7 @@ PyUnicode_DecodeUnicodeEscape(const char *s,
     _PyUnicodeWriter writer;
     const char *end;
     char* message;
+    int ret;
     Py_UCS4 chr = 0xffffffff; /* in case 'getcode' messes up */
     PyObject *errorHandler = NULL;
     PyObject *exc = NULL;
@@ -6022,6 +6023,11 @@ PyUnicode_DecodeUnicodeEscape(const char *s,
                 goto error;
             }
             else {
+                /* Deprecate invalid escape sequences */
+                ret = PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
+                                       "invalid escape sequence '%c'", c);
+                if (ret < 0)
+                    goto onError;
                 WRITECHAR('\\');
                 WRITECHAR((unsigned char)s[-1]);
             }