Occasionally I find it handy to tentatively do a rebase or similar, where I'm confident that the rebase itself will succeed but not so confident that the result will be something that I want to keep. In that case, making a branch at the current commit (using "git branch new-undo-point") will leave an easy way to return to the current state as if the rebase never happened.
Recently, in explaining this to a colleague who's relatively new to git, I think the "aha" moment came in realising that if you rewrite the history leading to commit X, you're really making new commit Y in addition to the old commit X - you're not changing X in to Y. Leaving a branch pointing at X is enough that git will keep it around after the history rewriting, in case you decide to do something with it later.
