Version 2.1 final release for Windows (using Idle)
Windows 98 2nd edition

f=open('c:\\My Documents\\textfile.txt', 'a')
f.write('hello file\n')
f.readlines()

raises the following error - it probably should.

Traceback (most recent call last):
  File "<pyshell#6>", line 1, in ?
    f.readlines()
IOError: [Errno 9] Bad file descriptor

However, the file gets a bunch of garbage appended to 
it.  In some cases, the entire contents of the GUI 
window gets appended.   

In Redhat 6.2 (also with Version 2.1 Final)the same 
code just erases the file.