This only seems to be a problem under Windows.
From a Portable Python discussion:
I am using Portable Python 1.1, python 3.0.1.
I am trying to set up a .bat file file to easily start IDLE. But IDLE
is throwing an error and failing some of the time.
It only happens if IDLE is set up to start in the edit mode. (Which I
prefer.) And then it only throws the error the first time a file is
opened. It doesn't seem to matter what kind of file is opened. The
other symptom is that a new line is inserted at the top of the file
that is opened.

The error is (I'm typing it by hand):
Exception in Tkinter callback
Traceback (most recent call last):
 File "E:\py30\App\lib\tkinter\__init__.py", line 1399, in __call__
   return self.func(*args)
 File "E:\py30\App\lib\idlelib\MultiCall.py", line 174, in handler
doafterhandler.pop()()
 File "E:\py30\App\lib\idlelib\MultiCall.py", line 221, in <lambda>
doit = lambda: self.bindedfuncs[triplet[2]][triplet[0]].remove(func)
ValueError: list.remove(x): x not in list

I can work around this with a batch file that uses python.exe, not
pythonw.exe, but it leaves an annoying command shell window open.

I also tested IDLE under a normal python installation and the same thing happens, so, sorry, it is not a PP thing. There must be a bug in IDLE under py 3.0. I'll file this with python.org.