I'm always amused by these because it signals the information source wasn't that important. Why does it matter where the phrase came from in most contexts?
Unfortunately, I've found that even with things I have fully read, I sometimes mix them up and confuse what came from where. There's just not enough distinction sometimes and it smears into a blob.
It's not so much what from where, but if you don't know the context, how are you going to be sure that you're not unintentionally libeling your client.
I'm aware that meanings change, and culture has its own uses for expressions. But the source is there for you to examine, there's no real excuse to use it wrong. We're not (yet) at the point where we have an oral culture tradition, but we're getting damn close.
Unfortunately, I've found that even with things I have fully read, I sometimes mix them up and confuse what came from where. There's just not enough distinction sometimes and it smears into a blob.