This is somewhat similar to #905389 which has already been fixed.

If you run the following two lines,

def gen(): raise UnicodeDecodeError
''.join(gen())

then instead of UnicodeDecodeError, you get:
TypeError: function takes exactly 5 arguments (0 given)

I found this bug in Python 2.3.5, 2.4.4 and 2.5.1 on Windows and Linux.

The bug appears exactly for UnicodeDecodeError,
UnicodeEncodeError, UnicodeTranslateError; all other exceptions work as expected. You can verify this with the following program:

import exceptions

def gen(e): raise e

for e in dir(exceptions):
    e = getattr(exceptions, e)
    if type(e) != type(Exception):
        continue
    try:
        ''.join(gen(e))
    except BaseException, f:
        e = e.__name__
        f = f.__class__.__name__
        if e != f:
            print e, f

This has nothing to do with str.join(). The following script raises the same exception:

def gen():
   raise UnicodeDecodeError()

gen()

The problem is exactly what the exception message states: the UnicodeDecodeError constructor expects five arguments. The following version raises the expected UnicodeDecodeError:

def gen():
   raise UnicodeDecodeError('ascii', 'bytes', 0, 1, 'ouch')
gen()

Thanks for the quick analysis. You're completely right, it has nothing to do with ''.join(). You get the same error if you simply

raise UnicodeDecodeError

which I think is a bit confusing nevertheless. I had naively assumed that you can instantiate every Exception with no or a single string argument only, so I did not consider this could be the cause. Sorry.