Status: Doesn't actually count as a fail; just displays the zero-lemmings-saved postview text. Since a level with a zero save requirement isn't going to happen outside of test situations, no need to worry about this.
I discovered this while looking at the ClimberGeneral.lvl test map. The number of required lemmings is set to 0.
However. if I either quit the level (without saving any), nuke all lems, or let them all die, the level still fails. I think the intended behavior should be that the level passes, even when you don't save any lemmings in a 0-lem save requirement.
I know some of these bugs/suggestions I'm reporting are minor/unimportant so far, but they're just things I want to point out anyway so I won't forget.
Note that only the message is wrong. If you encounter such a level in a pack, you will still pass onto the next level.
No worries - even if we decide "this doesn't need to be fixed"; it's better to report a possible issue and have it declared not a problem, than for one that really is a problem to go unreported. :)
In regards to this one, the reason this happens is quite simple, and it comes down to the priority of postview texts. After the saved-all message, the next highest priority is the saved-none. So therefore, it doesn't even bother to check how close you are to the requirement; it just sees "you haven't saved all lemmings, you have saved zero lemmings, display the saved zero message". And indeed, as Nepster mentions, it doesn't actually prevent passing. Since a zero save requirement is unlikely outside of test / demonstration maps for obvious reasons, I don't think we need to worry about this one. :)