imaplib.IMAP4_stream subprocess is opened unbuffered but ignores short reads when reading the message body.  Depending on timing, message body size and kernel pipe buffer size and phase of the moon and whether you're debugging the thing or not... It can fail to read the entire message body before wrongly assuming it has and attempting to read the terminating b')\r\n' of the IMAP protocol.

Bug discovered during a debugging session at the PyCon 2013 Python 3 Porting Clinic BOF.