There's a bug in python's zlibmodule.c that makes it raise SystemError whenever it tries to decompress a chunk larger than 1GB is size. Here's an example of this in action:

dmitriev@...:~/moo/zlib_bug> cat zlib_bug.py 
import zlib

def test_zlib(size_mb):
    print "testing zlib with a %dMB object" % size_mb
    c = zlib.compressobj(1)
    sm = c.compress(' ' * (size_mb*1024*1024)) + c.flush()
    d = zlib.decompressobj()
    dm = d.decompress(sm) + d.flush()

test_zlib(1024)
test_zlib(1025)

dmitriev@...:~/moo/zlib_bug> python2.6 zlib_bug.py 
testing zlib with a 1024MB object
testing zlib with a 1025MB object
Traceback (most recent call last):
  File "zlib_bug.py", line 11, in <module>
    test_zlib(1025)
  File "zlib_bug.py", line 8, in test_zlib
    dm = d.decompress(sm) + d.flush()
SystemError: Objects/stringobject.c:4271: bad argument to internal function

dmitriev@...:~/moo/zlib_bug>

A similar issue was reported in issue1372; however, either this one is different, or the issue still exists in all versions of Python 2.6 that I tested, on both Solaris and Mac OS X. These are all 64-bit builds and have no problem manipulating multi-GB structures, so it's not an out-of-memory condition:

dmitriev@...:~/moo/zlib_bug> python2.6   
Python 2.6.1 (r261:67515, Nov 18 2009, 12:21:47) 
[GCC 4.3.3] on sunos5
Type "help", "copyright", "credits" or "license" for more information.
>>> len(' ' * (6000*1024*1024))
6291456000
>>>