I have found a few cases when I try to answer someone's question based on my best understanding of what they are asking. Sometimes, I have made some assumptions based on their vague descriptions, or just based on speculation as to the source of the problem. Occasionally, these answers are based on wrong assumptions (shocking, I know). What I have found in some of these cases is that I get downvoted for these reasonable-yet-not-applicable answers. This has driven me to provide all my answers which are based on any kind of assumption as comments first. This works okay, but I've seen some discussion here on meta which seems to discourage posting answers as comments. So which is it?
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Don't post speculative answers.
Posting answers as comments isn't what comments are for, if you aren't sure what the question is about, post a comment and ask for clarifications. There is a ton of unanswered questions you could be answering instead of trying to guess what the OP means and providing wrong answers when you fail to do so.
First off: there's no possible way you can do this "safely" if you're guessing. Accept that. If you can't live with the possibility of being wrong, follow Yannis's advice and stick to answering less ambiguous questions.
If you do want to gamble, I have some recommendations that can help to make this a bit less stressful for everyone involved:
You should request clarification from the poster in comments, and build an appropriate answer based on that feedback.
If the question so vague that it requires guesses on your part to answer, it's not a good question yet. Make the person who asks provide the required detail before you attempt to answer.
Note that this doesn't always work. If the person doing the asking doesn't provide sufficient details to make his question answerable, just walk away (possibly casting a close vote as you go).