in git, what ways are there to "lose" a commit in a way that you CAN'T recover using the reflog (so that you need to iterate over every single commit in the repository if you want to find it?

The only ways I know (using git's normal tools) are:

* using `git stash drop` or `git stash pop` to drop a stashed commit
* waiting 90+ days to try to recover the commit (so that it expires from the reflog)
* explicitly deleting the reflog in some way (rm -rf .git, git reflog expire, etc)

@b0rk filter-branch? Not sure that's what you're looking for, because it's more intentional, than "lose".
@cmw @b0rk Came here to mention filter-branch and plumbing (vs porcelain) commands that one would rarely use unless writing your own tools.