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Note that this does not just affect MacOSX, this even affects Linux (although the behaviour on the default filesystems is different).
On Linux the default is case-sensitive filesystems, but you can use network filesystems that have case-insensitive behavior, for example by using a CIFS share. 
This means that /mnt/myserver-share/foo/bar and /mnt/myserver-share/FOO/BAR may or may not refer to the same "object": there aren't the same if /mnt/myserver-share is a regular Linux filesystem but will be when /mnt/myserver-share is a mount from a CIFS server.
In short: os.path.normcase is hopelessly naive in its assumptions about system behavior.