This problem happens when I unpack a file from a 200+ MB zip archive as follows:

with zipfile.ZipFile(archive) as z:
    data = b''
    with z.open(filename, 'rU') as f:
        for line in f:
      	    data += line


I cannot reduce it to a test case suitable for posting here, but the culprit is the following code in zipfile.py:

    def peek(self, n=1):
        """Returns buffered bytes without advancing the position."""
        if n > len(self._readbuffer) - self._offset:
            chunk = self.read(n)
            self._offset -= len(chunk)

See http://hg.python.org/cpython/file/81f8375e60ce/Lib/zipfile.py#l605

The problem occurs when peek() is called on the boundary of the uncompress buffer and read() goes through more than one readbuffer.  The result is that self._offset is smaller than len(chunk) leading to a non-sensical negative self._offset upon return from peek().

This problem does not seem to appear in 3.x since 028e8e0b03e8.

Does this patch fix a bug?

It does!

New changeset 8b097d07488d by Serhiy Storchaka in branch '2.7':
Issue #20048: Fixed ZipExtFile.peek() when it is called on the boundary of
http://hg.python.org/cpython/rev/8b097d07488d

Than you for your report and irrefragable analysis.