diff -r 52f68c95e025 Lib/idlelib/configHandler.py
--- a/Lib/idlelib/configHandler.py	Thu Jan 26 08:47:27 2012 -0600
+++ b/Lib/idlelib/configHandler.py	Sat Jan 28 14:36:59 2012 -0600
@@ -19,6 +19,7 @@
 """
 import os
 import sys
+import tkinter
 
 from idlelib import macosxSupport
 from configparser import ConfigParser, NoOptionError, NoSectionError
@@ -521,7 +522,57 @@
                 if v != v2:
                     result[k] = v2
 
-        return result
+        return self.ValidateKeys(result)
+
+    def ValidateKeys(self, keyset):
+        """ Go through the dictionary of keys and make sure each is valid """
+        tk = tkinter.Tk()
+        tk.withdraw()
+
+        total_invalid = []  
+        for event, keylist in keyset.items():
+            invalid = []
+            for key in keylist:
+                try:
+                    tk.bind(key, lambda e: "break")
+                except:
+                    invalid.append((event, key))
+            for _event, key in invalid:
+                # IDLE's architecute as of 2012-01-28 calls GetCurrentKeySet for each loaded extension,
+                # hence, this message will print several times.
+                print('  IDLE Configuration Error: removing invalid key for event %s: %s' % (_event, key),
+                      file=sys.stderr)
+                keylist.remove(key)
+
+            total_invalid.extend(invalid)
+
+        self._keybinding_error(total_invalid)
+
+        tk.destroy()
+        return keyset
+
+    def _keybinding_error(self, invalid):
+        if not hasattr(self, "known_invalid"):
+            self.known_invalid = []
+        known_invalid = self.known_invalid
+
+        # Give the user an error message based on newly found invalid keybindings
+        new_invalid = [i for i in invalid if i not in known_invalid]
+        known_invalid.extend(new_invalid)
+
+        if new_invalid:
+            #print('Newly invalid:', new_invalid)
+            msg = ['There are invalid key bindings:', '']
+            for ev, key in new_invalid:
+                while ev[0] == '<' and ev[-1] == '>':
+                    ev = ev[1:-1]
+                msg.append('Action:%s' % ev)
+                msg.append('Key:%s' % key)
+                msg.append('')
+
+            msg.extend(['Please reconfigure these bindings in the configuration dialog.'])
+            tkinter.messagebox.showerror(title='IDLE: Invalid Key Binding Error',
+                                   message='\n'.join(msg))
 
     def GetKeySet(self,keySetName):
         """