I came across this question and answer earlier today. The question was good, and quickly received an answer. To summarize, a snippet from the question:
Can I always replace let go with leave go?
And the answer that was first posted, in its entirety:
Leave go for let go would not be recognized in America. It must be let go here.
The asker then responded with a polite comment, pointing out that they were only concerned with British English (and had tagged the question as such), therefore making the answer invalid.
Now here's what I'm thinking: I only noticed that the question had been tagged british-english when I read that comment. Clearly the person who posted that answer didn't notice the tag either, or of course they wouldn't have posted an answer referring only to American English.
It seems to me, then, that if a tag changes the answer to your question, it should be mentioned in the actual question text also. In the interests of collecting quality answers that actually answer the question, I think it would help if all relevant information is in the question text itself, not just in tags. Tags are useful for categorizing, but the question should speak for itself without them, in my opinion. Thoughts?
(As a side note: it almost seems like this phenomenon is the opposite of a meta-tag. A meta-tag is a tag we want to get rid of, because it doesn't categorize the question in a useful way, or add anything to it. This is a tag that we do want, but really need to have in the question, because it's not implied by the question alone--the answerer needs this information to respond.)