Kees Cook made a kernel documentation commit with the same first 12 characters as the initial commit in the repository, serving as a test commit. This has not been included in the upstream Linux tree to avoid breaking various tools. Commit-ID collisions were previously explored by LWN. Some users mistakenly referred to Kees as Case. Namespace collision is another issue highlighted. (Note: Controversy around the mix-up of Kees and Case, uniqueness in the commit ID similarity, and the relevance of namespace collision)
https://lwn.net/Articles/1003797/