diff -r cb2e3188832b Lib/_pyio.py
--- a/Lib/_pyio.py	Tue May 31 17:11:26 2011 -0500
+++ b/Lib/_pyio.py	Tue May 31 20:56:12 2011 -0400
@@ -165,8 +165,8 @@
     text = "t" in modes
     binary = "b" in modes
     if "U" in modes:
-        if writing or appending:
-            raise ValueError("can't use U and writing mode at once")
+        if writing or appending or updating:
+            raise ValueError("mode U cannot be combined with 'w', 'a', or '+'")
         reading = True
     if text and binary:
         raise ValueError("can't have text and binary mode at once")
diff -r cb2e3188832b Lib/test/test_file.py
--- a/Lib/test/test_file.py	Tue May 31 17:11:26 2011 -0500
+++ b/Lib/test/test_file.py	Tue May 31 20:56:12 2011 -0400
@@ -139,7 +139,7 @@
 
     def testModeStrings(self):
         # check invalid mode strings
-        for mode in ("", "aU", "wU+"):
+        for mode in ("", "aU", "wU+", "U+", "+U", "rU+"):
             try:
                 f = self.open(TESTFN, mode)
             except ValueError:
diff -r cb2e3188832b Misc/NEWS
--- a/Misc/NEWS	Tue May 31 17:11:26 2011 -0500
+++ b/Misc/NEWS	Tue May 31 20:56:12 2011 -0400
@@ -10,6 +10,9 @@
 Core and Builtins
 -----------------
 
+- Issue #2091: open() accepted a 'U' mode string containing '+', but 'U' can
+  only be used with 'r'.
+
 - Issue #12225: Still allow Python to build if Python is not in its hg repo or
   mercurial is not installed.
 
diff -r cb2e3188832b Modules/_io/_iomodule.c
--- a/Modules/_io/_iomodule.c	Tue May 31 17:11:26 2011 -0500
+++ b/Modules/_io/_iomodule.c	Tue May 31 20:56:12 2011 -0400
@@ -371,12 +371,11 @@
 
     /* Parameters validation */
     if (universal) {
-        if (writing || appending) {
+        if (writing || appending || updating) {
             PyErr_SetString(PyExc_ValueError,
-                            "can't use U and writing mode at once");
+                            "mode U cannot be combined with 'w', 'a', or '+'");
             return NULL;
         }
-        reading = 1;
     }
 
     if (text && binary) {