I posted an example a few days ago. I see a patch on the list, take it, modify it to improve _some_ parts of it, but it's still not "finished" and I'm still don't agree that it should be committed. But at least it's a bit better now.
In such a case I'd say I should sign-off (as I modified the patch), but I don't want to Ack it.
Yeah. But the only "acked-by"s that have any sense tracking are those on the final version of the patch, i.e. the version that gets checked in, so the point is moot.