There are some attempts at posts at some times that elicit this error message. But at other times embedding urls that point to images or to pages containing images works just fine.
Lots of users have questions about these events. And about the best answer our homegrown power users have is “It does that sometimes” and “You’re doing something wrong.” Which is pretty unsatisfying.
It would be nice if @codinghorror or one of the other insiders can tell us exactly what that message is trying to say. What scenarios trigger it and what’s the workaround? What are people doing wrong when they provoke this message?