I think there's more corners out there than most people would give credit to? Just off the top of my head: files in the index (but maybe this isn't "weird enough"), rebasing but paused, rebasing with conflicts, merge with conflicts, cherry-picking but conflicts, middle of a bisect with all the state that implies, alternate objects dirs, alternate working dirs, submodules and all of their weirdness, and a "bare" repo.
Heck, had my PS1 return an error this week after I created a separate working dir for a repo and cd'd into it. Did you know .git can be a normal file? I didn't when I wrote my PS1.
I knew .git can be a normal file because of worktrees. But most of the weird states have to do with the working tree not the repository. Even rebasing isn’t weird as far as the file formats go: it just is replaying commits on top of a new base commit. Since my goal was basically to implement enough of git to serve files from a git repository as a website, the actual task was fairly small.
Heck, had my PS1 return an error this week after I created a separate working dir for a repo and cd'd into it. Did you know .git can be a normal file? I didn't when I wrote my PS1.