[More info from Ronald Oussoren]

This is a bug in Tk: 

>>> root = Tkinter.Tk()
Thu May 13 20:45:13 Rivendell.local python[84887] <Error>: kCGErrorFailure: Set a breakpoint @ CGErrorBreakpoint() to catch errors as they are logged.
_RegisterApplication(), FAILED TO establish the default connection to the WindowServer, _CGSDefaultConnection() is NULL.
>>> 2010-05-13 20:45:16.751 Python[84887:d07] An uncaught exception was raised
2010-05-13 20:45:16.762 Python[84887:d07] Error (1002) creating CGSWindow
2010-05-13 20:45:16.955 Python[84887:d07] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Error (1002) creating CGSWindow'
*** Call stack at first throw:
(
	0   CoreFoundation                      0x00007fff85e31d24 __exceptionPreprocess + 180
	1   libobjc.A.dylib                     0x00007fff860000f3 objc_exception_throw + 45
	2   CoreFoundation                      0x00007fff85e31b47 +[NSException raise:format:arguments:] + 103
	3   CoreFoundation                      0x00007fff85e31ad4 +[NSException raise:format:] + 148
	4   AppKit                              0x00007fff84614aba _NSCreateWindowWithOpaqueShape2 + 473
	5   AppKit                              0x00007fff845a9055 -[NSWindow _commonAwake] + 1214
	6   AppKit                              0x00007fff845c6d3d -[NSWindow _makeKeyRegardlessOfVisibility] + 96
	7   AppKit                              0x00007fff845c6cb2 -[NSWindow makeKeyAndOrderFront:] + 24
	8   Tk                                  0x000000010075b86c XMapWindow + 155
	9   Tk                                  0x00000001006ca6d0 Tk_MapWindow + 89
	10  Tk                                  0x00000001006d35e6 TkToplevelWindowForCommand + 2658
	11  Tcl                                 0x00000001006300d3 TclServiceIdle + 76
	12  Tcl                                 0x00000001006162ce Tcl_DoOneEvent + 329
	13  _tkinter.so                         0x0000000100595683 Tkapp_CallDeallocArgs + 277
	14  readline.so                         0x00000001001f1f9a initreadline + 1280
	15  Python                              0x00000001000076a1 PyOS_Readline + 239
	16  Python                              0x0000000100008a57 PyTokenizer_FromString + 1322
	17  Python                              0x00000001000090a0 PyTokenizer_Get + 154
	18  Python                              0x0000000100005698 PyParser_AddToken + 1018
	19  Python                              0x00000001000a2320 PyParser_ASTFromFile + 146
	20  Python                              0x00000001000a443f PyRun_InteractiveOneFlags + 345
	21  Python                              0x00000001000a4615 PyRun_InteractiveLoopFlags + 206
	22  Python                              0x00000001000a4685 PyRun_AnyFileExFlags + 76
	23  Python                              0x00000001000b0286 Py_Main + 2718
	24  Python                              0x0000000100000e6c start + 52
	25  ???                                 0x0000000000000001 0x0 + 1
)
terminate called after throwing an instance of 'NSException'
Abort trap


This is running /usr/bin/python in a session as a user that doesn't have access to the GUI.  The text above says that there is an uncaught ObjC exception, caused by the lack of a connection to the window server. Tk should have converted that to its own style of errors but didn't.