Frequently I see questions where the OP has basically come to the conclusion: "If only I knew how to do Z, then I could solve my problem," then basically asks "How do I do Z?" on SO (or pick another site). Often, it turns out that "doing Z" is impossible or impractical (see this example) and you get the sense that if only the user had described their actual problem, you might be able to help them.
Is it advisable to encourage people to describe the context of their question as well as the specific question as well? How can we encourage people to do this? I usually comment, but there is often a lag before the question gets updated with additional information, leaving some answers as irrelevant once the real question comes to the fore. People tend not to read instructions, so perhaps there isn't a good way to catch this any earlier. Thoughts?