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codecs.readlines() does not read to the end of the file if called after codecs.readline(). 
Skimmed through tasks containing codecs in title and could not find a candidate that sounded identical.
Repro follows:
$ cat sample_text.txt
Subject: Incorrect email address
 
RATER EMAIL rejected an invitation from SUBJECT NAME <SUBJECT EMAIL> in
the PROJECT TITLE project. Notification was sent to <RECIPIENT EMAIL>,
but the email address was no longer valid.
$ python
Python 2.7.3 (default, Sep 26 2012, 21:53:58) 
[GCC 4.7.2] on linux2
>>>
>>> import codecs
>>>
>>> # No problem if readlines() are run at the beginning:
>>>
>>> f_in = codecs.open('sample_text.txt', 'rb', 'utf-8')
>>> f_in.readlines()
[u'Subject: Incorrect email address\n', u'\n', u'RATER EMAIL rejected an invitation from SUBJECT NAME <SUBJECT EMAIL> in\n', u'the PROJECT TITLE project. Notification was sent to <RECIPIENT EMAIL>,\n', u'but the email address was no longer valid.']
>>> f_in.close()
>>>
>>> # Let us try to read the first line separately,
>>> # and then read the remainder of the file:
>>>
>>> f_in = codecs.open('sample_text.txt', 'rb', 'utf-8')
>>> f_in.readline()
u'Subject: Incorrect email address\n'
>>> f_in.readlines()
[u'\n', u'RATER EMAIL rejected an invitation fro']
The first readlines() does not read to the end. Subsequent readlines() returns what's left to read.
sample_text.txt attached.

Confirm for 3.x.
See also issue8260, issue12446, and issue14475.

Thank you! This is indeed a duplicate of issues #8260, #12446.