This most likely will never happen unless someone specifically force the
last modification time of a file in some way (if I remember correctly there
should be a Windows utility which does that).

The test case in attachment shows how to emulate the issue by forcing
os.path.getmtime to return an unexpected value producing the following
traceback message:

test_unforseen_mdtm_event (__main__.TestFtpFsOperations) ... Traceback
(most rec
ent call last):
  File "C:\Python25\lib\asyncore.py", line 68, in read
    obj.handle_read_event()
  File "C:\Python25\lib\asyncore.py", line 390, in handle_read_event
    self.handle_read()
  File "C:\Python25\lib\asynchat.py", line 137, in handle_read
    self.found_terminator()
  File "c:\python25\lib\site-packages\pyftpdlib\ftpserver.py", line 1776,
in fou
nd_terminator
    method(arg)
  File "c:\python25\lib\site-packages\pyftpdlib\ftpserver.py", line 2662,
in ftp
_MDTM
    lmt = time.strftime("%Y%m%d%H%M%S", 
time.localtime(lmt))
ValueError: (22, 'Invalid argument')