In Leopard, many man pages (the documentation for Mac OS X’s UNIX commands) are compressed with gzip. that is a very good notion but, whether you upgraded to Leopard from Tiger, the old man pages are left in place by the installer (this is a bug).

So many man pages are there in two versions: the old uncompressed one from Tiger, and the new compressed one from Leopard. The problem is that when man searches for a given

page, it finds the uncompressed one first and displays it, thus showing you outdated knowledge. To solve the problem, the old man pages must be removed. I made a shell script that removes everything in /usr/share/man that is not installed by Leopard (I made the script by actually installing Leopard on a blank disk and comparing with my installed system). Here’s the command to build the script:
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Original post by Mike Schramm