My ears perked up when I heard that it involved renames on OSX. I don't know about the exact issue he had, but I recently found out the hard way that OSX's HFS+ is a case-insensitive* filesystem. You can get subtle issues by importing multiple files with the same case (such as "README.txt" and "ReadMe.txt") into the same repository; this isn't specific to git.
I had a similar issue with Perforce on Windows - Perforce was case sensitive, Windows wasn't, and thanks to CamelCase, there were two files that had the same letters but different casing. (I don't remember the names.)
* Technically, "case-insensitive but case-preserving", which in practice seems to mean, "case-sensitive, except when you need it to be".