The editor needs an undo button!
If one person makes a mistake in the editor such as moving a terrain piece or object by accident, they have no way of moving it back to where it exactly belongs. What if they move that terrain/object, and later they don't know where to put it back?
Also, if I were editing a Lemmings Plus Recycle Bin level but I don't want to move any of the terrain pieces or objects by accident (i.e. ruin the work of the original creator), this would be very beneficial. This is also beneficial for collaboration levels and tag-team contests.
If Microsoft Word can have it, Microsoft Paint can have it, and if you can backwards frameskip to fix a mistake in a level, I don't see why it would be any different for the NeoLemmix Editor.
A redo button wouldn't hurt either. Also, it would be beneficial if one can undo/redo as many times as they would like.