I specifically used the formulation in the title from here, the only place I found to deal with this. But I do not know, what to take from it.
This question arose from comments on this answer. I know the "check my solution" questions are considered off topic. However, I think this is different. The solution is known to be wrong and we are seeking only the mistake, that must necessarily be there. Of course the mistake might be only forgotten minus sign, or the mistake might lead to clarification of important conceptual misconception and in this case, it would be helpful to broader audience. Sadly, as the mistake is unknown, better question than just "spot the mistake" cannot be asked.
It seems to me such question can be rightly heavily downvoted - as it is in general uninteresting - but I do not think it should be considered off-topic.
What is the official stance of this site?