Today I had a Git experience :

It has been '0' days since I hit a mysterious Git error out of nowhere, during a completely routine 'git pull' in a repository that's identical with upstream:

error: fetching ref refs/remotes/origin/master failed: incorrect old value provided

What should I do? What's wrong? Good luck figuring it out. Fortunately this is just a tracking repository, so maybe the correct answer is 'delete and re-clone'.

This turned out to be my own fault ( as suggested by a helpful Fediverse denizen ). I have copies of this repository on several hosts, and because I want to read every commit message in it, I try to update all of those repositories at the same time, getting the same new commits in each. This time around I accidentally opened two windows on the same host and didn't notice, so when I ran 'git pull' in each of them at the same time, they stepped on each other somehow.

(I run the 'git pull' at the same time in each copy of the repository to maximize the odds that they'll pull the same set of changes. Pulling the same set of changes makes it easy to read all of the commit messages only once. This is all a bit awkward but as far as I know it's the easiest way to maintain multiple independent copies of an upstream yet read all of the new commit messages only once.)

This isn't the first time I've accidentally done two overlapping 'git pull' operations on the same repository. I think it's the first time I hit this error and also the first time I didn't notice what the real problem was right away. Having stubbed my toe on this more than once, this time rather vividly, hopefully in the future I'll remember to check for this cause if I have weird things happen during Git operations.

Git has some locking around the Git index, which you can discover if Git commands start complaining that an 'index.lock' file already exists. I believe the general discussion of this is in api-lockfile , and if I'm reading it right, 'index.lock' is not just the lock file, it's the new version of the index file. Lock files are apparently also used for at least the commit graph file , and the git-config manual page has a tantalizing list of various lock timeouts. However, there evidently isn't enough locking to stop accidents completely, especially for multi-step operations like 'git pull' (which is actually 'git fetch' plus a fast-forward update done somehow).

(Based on this , I think Git references like 'HEAD' can also be locked; also .)