(tested and verified on Windows and Solaris SPARC)

Running this code in Python 2.4, 2.5 or 2.6 (all minor versions)
produces garbage.

f=open("anyfile","w")
f.write("garbage")
f.readline()

Mac OS X and Linux appear to simply throw an "IOError: [Errno 9] Bad
file descriptor" exception, while using a read method without writing
first produces the same exception on Windows and certain versions under
Solaris.

Under Solaris, it is further possible to segfault the interpreter with
this code:
f=open("anyfile","w")
f.read()

In the former case, it appears as if the data is simply read from the
disk block containing the file. In the latter, I have no clue what is
going on.

In Python 3.0, file objects opened with "w" don't even support any .read
methods, so this does not affect Py3k.