alt Hacker NewsThe well known XY problem[1].
I spent years on IRC, first getting help and later helping others. I found out myself it was very useful to ask such questions when someone I didn't know asked a somewhat unusual question.
The key is that if you're going to probe for Y, you usually need to be fairly experienced yourself so you can detect the edge cases, where the other person has a good reason.
One approach I usually ended up going for when it appeared the other person wasn't a complete newbie was to first explain that I think they're trying to solve the wrong problem or otherwise going against the flow, and that there's probably some other approach that's much better.
Then I'd follow up with something like "but if you really want to proceed down this rrack, this is how I'd go about it", along with my suggestion.