struct.pack('L', -1) raises a DeprecationWarning since Python 2.5, as it
should.  However, it also returns a different (and nonsensical) result
than Python <= 2.4 used to: it returns '\x00\x00\x00\x00' instead of
'\xff\xff\xff\xff'.

This might lead the zipfile module of release25-maint (the version >=
2.5.2) to produce buggy zip files.  The -1 value can come as the
header_offset field, which will then be packed as an all-0 string
instead of an all-ff string in the zip file headers.

Given the DeprecationWarning I would classify this as low priority. 
However, given that the stdlib module zipfile relies on this feature in
release25-maint, it should probably really be fixed.