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I was able to get some error output with the code of the OP. However, I only saw the "opposite" message, such as:
Retrieval of 'ftp://ftp.gnome.org/pub/debian/dists/stable/main/source/Sources.bz2' failed with error: [Errno ftp error] 200 Switching to Binary mode.
I had to change the used FTP server and path because I had problems connecting to that server.
Even with the guidelines by the OP, I could trigger the bug only occasionally.
I converted the script to be executable with manual intervention (see attachment). Still, sometimes both files are also successfully downloaded in the second iteration (i. e. the "outer" iteration which downloads both files).
These results are from a test with Python 2.6.6. I plan to continue testing with the tip of the Python 2.7 branch.