When email.FeedParser.BufferedSubFile sees "\r" at the end of the pushed-in data, it assumes that it is a Macintosh-style line terminator.  Instead, it should request more data, to ensure that the next character is not "\n", which would make it a Windows-style line terminator.  This affects email.message_from_file, which reads in the data in 8192 byte chunks.  The following code demonstrates this:

====================================
from StringIO import StringIO
from email.FeedParser import \
    BufferedSubFile, NeedMoreData

fp = StringIO( "1\r\n10\r\n100\r\n"
               "1000\r\n10000\r\n" )
bsf = BufferedSubFile( )
while True:
    data = fp.read( 3 )
    if not data:
        break
    bsf.push( data )
    for line in bsf:
        if line is NeedMoreData:
            break
        print repr( line )
bsf.close()
====================================

The output is:
====================================
'1\r\n'
'10\r'
'\n'
'100\r\n'
'1000\r\n'
'10000\r'
'\n'
====================================

File Added: BufferedSubFileBug.py

Confirmed on trunk.

Attached a patch containing a unit test based on Sye van der Veen's
example and a solution for this problem.

Written against Python 2.6 (trunk), r74214, which was current at the
time of submission.

This is my first patch, so please let me know if I did something wrong
or overstepped bounds by not noticing that this was assigned until after
writing this fix.

Confirmed in trunk and all current branches (r74225: 2.6, 2.7, 3.1, 3.2).

Patches for all four active versions will be added momentarily.

Note: my submission yesterday was mistagged, claiming to be for trunk
while it was really for 2.6, which is what this bug was actively marked
with at the time.

Attached a patch containing a unit test based on Sye van der Veen's
example and a solution for issue 1721862.

Written against Python 2.6, r74225.

Attached a patch containing a unit test based on Sye van der Veen's
example and a solution for issue 1721862.

Written against Python 2.7, r74225.

Attached a patch containing a unit test based on Sye van der Veen's
example and a solution for issue 1721862.

Written against Python 3.1, r74225.

Attached a patch containing a unit test based on Sye van der Veen's
example and a solution for issue 1721862.

Written against Python 3.2, r74225.

This seems to be a duplicate of issue 1555570, which has a simpler patch.