I was thinking about the recent question "Why is making a white laser so difficult?"
A couple of days back (when I first saw it) if I recall correctly, I left my usual off-topic-because-it's-engineering comment under it: "This may receive better answers on engineering SE." The comment has been deleted (probably by a mod), so I'm not even sure if the person who suggested engineering SE was actually me. But one of the other comments quotes my usual phrase "better answers", and I had left a flag on the question, so it's likely.
Looking at the present state of the question, I think it may actually be on-topic: Emilio Pisanty's answer (to the same question) successfully explains things without going into a discussion of engineering: he says that white light lasers can be made and gives an explanation of how they can be theoretically created by choosing appropriate gains or dyes. However, the answer by MrMins (which is presently the highest voted answer) says that the existing white lasers are purely conceptual and that the technology is not "practical" yet, and basically answers the question by saying "White lasers aren't common because of it's presently difficult to make them run on batteries", which signals engineering at full volume.
To me the question "Why is it difficult to make this?" is definitely engineering if there are practical considerations. Physics in such cases is (IMO) "Is it theoretically possible for such a thing to exist?", or "How do white lasers work?". Furthermore, the question, when posted on engineering SE, received a negative score (though only -1), and the answer there is definitely engineering-ish: it says that one of the ways of making white lasers is possible but is "a very difficult engineering problem", and gives you a link to an article about how some people created a single-crystal white laser.
My summary: the answers on Physics SE made the question seem to be on-topic, but the identical question on engineering SE received an engineering-ish answer. What do we do with such questions?
I don't have a problem with the fact that a moderator deleted the comment. There's a possibility that it's misleading: I believe the question is off-topic, but apparently most people disagree, and that's fine. If the comment causes an unnecessary confusion, it should go.
In this case, all I knew about the answer to the question was what a quick google search told me: white light lasers definitely exist and have some applications in medical stuff. To me, this strongly suggests engineering complications as the answer to the question assuming the premise that they're hard to make is correct
I'm deliberately not using the specific-question tag here. I don't want the discussion to be focused upon this question. This is probably applicable to a large number of off-topic engineering questions: they could possibly receive satisfactory physics explanations in addition to engineering considerations