As I don't see any active challenge in here right now, I'll take the opportunity to revive this thread with a new (possibly somewhat difficult) one:
Design a level that, while requiring a constant amount of Lemmings to be saved, can only be solved if the amount of Lemmings that get released from the trapdoor(s) is a certain number smaller than 40, and cannot be solved if the amount of Lemmings is lower or higher than that number.
E.g. your level requires 10 lemmings to be saved, and it can only be solved if the amount of Lemmings in the level is set to 30, but not if it set to 10, 29, 31, 80 or any other value that is not 30, while no other changes are made.