Though the refspec isn't the first thing that comes to mind when thinking about the Git configuration, it is actually quite close. In a lot of the Git commands the refspec is used, but often implicitly, that is, the refspec is taken from the configuration file. If you don't remember setting a refspec configuration, you are probably right, but if you cloned the repository or added a remote, you'll have a section in .git/config, which looks something like the following (this is for the Jgit repository):

[remote "origin"]
        url = https://git.eclipse.org/r/jgit/jgit
        fetch = +refs/heads/*:refs/remotes/origin/*

The fetch line contains the configured refspec to fetch for this repository.