Suppose there is a bug in git causing developers commit incorrect changes to the repository so that sources checked out do not match what they have locally under same hash. I understand that hash protects structure of git objects, not checked out sources.
Does git have any safeguard mechanisms that make this less likely than one might think?
That would only happen if there is a SHA1 collision, meaning if the content SHA1 is the same as the one stored in the Git repo.
You can check that by computing yourself the SHA1 of a file you just checked out.
See also "How does git compute file hashes?".
The probability of a collision (two different content for the same SHA1) is very low: see "Why does Git use a cryptographic hash function?".
