Earlier this evening I tried to retweet something, and got this message:  

You have been blocked from retweeting this user's tweets at their request. 

If you don't believe me, here's a screen shot. 

All kinds of thoughts come up. 

1. I didn't know you could do that. 

2. Is it in the user interface? 

3. If not, how do you get this done? 

4. Why would someone want to do this? 

5. Retweeting seems pretty neutral. Not? 

Anyway, I did a Google search and found it has come up before, so I'm not the only one. But not very often, which seems to indicate there is no user interface for it. Very odd. 

BTW, "just blocking them" is not an answer, because a friend who I don't want to unfollow frequentlly RT's the person who has me blocked from RT'ing. If it gets annoying enough I guess I will have to unfollow the friend who I don't want to unfollow.  

Also, it's that his tweets are private, they aren't. I am not following him. Another friend is retweeting them. Unless their "privacy" includes this feature, they aren't private.  

In case it isn't obvious, Twitter is a morass of inconsistent rules about when it should and shouldn't show you stuff, and makes you block people when you have no interest in blocking them just to get another feature that comes along with blocking.