Semi-periodic reminder that the DM feature here is utterly broken and should not be regarded as "private" in any meaningful sense. And its semantics seem designed to violate the principle of least surprise.
Example: ANYONE mentioned in a message, not just tagged at the start, is a recipient. So if you send someone a DM that says "That @mattblaze guy is an ignorant moron", I get a copy.
I was reminded of this behavior just now.
@mattblaze
I agree. The feature doesn't seem to have been implemented well.
I've always referred to this kind of mistake as violating the "principle of least astonishment" ( #POLA). Wikipedia mentions the term you used, too.
https://en.wikipedia.org/wiki/Principle_of_least_astonishment
