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I can get a similar error from time to time, just try it a couple of
times and I believe you should hit it too.
When that error isn't thrown IDLE prints the traceback of a SystemExit
exception inside IDLE right before closing, so it may be hard to notice.
The error I am able to get (using 3.1rc2+ r73431):
*** Internal Error: rpc.py:SocketIO.localcall()
 Object: stderr 
 Method: <bound method PseudoFile.write of <idlelib.PyShell.PseudoFile
object at 0x8b6084c>> 
 Args: ('SystemExit: None\n',)
Traceback (most recent call last):
 File "/home/gpolo/python-dev/py3k/Lib/idlelib/rpc.py", line 188, in
localcall
 ret = method(*args, **kwargs)
 File "/home/gpolo/python-dev/py3k/Lib/idlelib/PyShell.py", line 1231,
in write
 self.shell.write(s, self.tags)
 File "/home/gpolo/python-dev/py3k/Lib/idlelib/PyShell.py", line 1214,
in write
 self.text.mark_gravity("iomark", "left")
AttributeError: 'NoneType' object has no attribute 'mark_gravity'
I'm not sure if it can't happen in python-trunk too, I see it throws an
exception when leaving IDLE but till now I never got an error message
after IDLE (almost) exited.