The MO file format specification reads:
A program seeing an unexpected major revision number should stop reading the MO file entirely
But polib doesn't pay attention to versions at all.
As a test-case I attached a MO file with a bogus major revision number. msgunfmt correcly rejects such a file:
$ msgunfmt messages.mo msgunfmt: file "messages.mo" is not in GNU .mo format
Yet polib opens it happily:
>>> import polib >>> polib.mofile('messages.mo') [<polib.MOEntry object at 0xf6fdc14c>]