Gittin 25: Retroactive Clarification

When a man wants to divorce a wife, and he has at least two, and he doesn't want to specify which one, and devises a plan for, let's say, whichever one comes to greet him first - the Gemara says, no. The get needs to be written in her name - they can't discover after the fact which wife was, lo and behold, the divorced one. But there many other circumstances where retroactive clarification does work.