Effectively nothing. By default, old, unreferenced data is only garbage collected after a month. If you screw up a rebase, the old commits are still there with the same hash they always had. You just need to point the branch at them again.
> ...or uncommitted working files
That's fair. If I'm unsure, I just stash or commit first. Anything that's in git can be retrieved.
Git makes using branches very easy and it is fairly quick with most operations. Once you understand what the commands are doing, you'll see that unless you do something specifically (like reset --hard with uncommitted changes, checkout a modified file etc.), you'll never lose work which has been committed in some branch or the other.
I tend to just do "in-between" commits very often and rebase to reorganise the commits (reorder, squash, reword) when I hit a (micro)milestone.
If your work has been committed and the repo was not just garbage collected, it is going to be easy to retrieve it from reflog even if you did something nasty with the branch and it appears that your commit has disappeared.
Git is the piece of software which I find very good in terms of what it does but it does it behind the worst user-interface I have ever encountered.
EDIT: minor clarifications and punctuation
`git clone https://repository.url` (download repository)
`git checkout -b issue_XXX` (create new branch with the name 'issue_XXX')
`vim README.MD` (edit a file)
`git add README.MD` (mark the file to be committed, `git reset README.MD` to unmark)
`git status` (see list of changed/marked files)
`git commit -m "Add Readme"` (create commit based on marked files)
`vim README.MD`
`git add README.MD`
`git commit -m "Fix Typo"`
`git checkout master` (switch to branch with name 'master')
`git merge --squash issue_XXX` (merge branch with name 'issue_XXX' into current branch (master), but stop before committing)
`git commit -m "Add Readme.MD"` (create a new commit)
`git push` (upload changes)
psa: `git reflog show stash` shows the stash list along w/ with their shorthashes for easy reference
Disclaimer, if you're compelled for none other than neurotic reasons to run `git gc` periodically like me, these old commits will get thrown out. I found this out the hard way :(