I just have to share this.
If anything, the discussion there is a perfect illustration of the maxim that leaving things unsaid, of just assuming that some things are understood, is about the worst thing one can do.
No matter how trivial, Some Things Need To Be Said.
Inscribe this on your foreheads (or write it on the palm of your hand).
Update: In rereading the thread I linked to above, it occurs to me that the commenter may have not referred to me at all about the “common knowledge” phrase, but to the well-known fact that president Obama is black. I don’t really know, but it’s such a good title that I thought I’d let it stand for now. When I get clarification, I’ll change it.