I'm not going to present data - just a personal anecdote. If someone else has done a systematic analysis, it would be a much better answer.
Are standards rising?
Here is a brief history of Skeptics.SE.
In the early days of the site, we were still exploring what our internal standards for answers should be. For example, This discussion about references shows ...
Your question eliminates all the possible situations which would deal with a good answer that has some flaws. Here is what one should do if they find a good answer which has some flaws.
Let's start with:
Upvoting the answer has a clear downside. Downvoting the answer is not terribly fair.
Ideally, a good question (even if it flaws) should be upvoted and ...
The problem here is balancing out what a user think is wrong, and another thinks is right. If there are no references, moderators can intervene.
If it's a grey area, I'd say leave a comment. Yeah, it won't work, but it doesn't matter. Your correction is there with the answer now.
Comments do influence votes, and sometimes authors actually... correct their ...
I think the second-last suggestion is best. You can write a small new answer which says something like,
One detail that X didn't mention in their otherwise-excellent answer was Foo.
Foo is referenced at Bar and it's relevant because of Baz.
I disagree with Oddthinking's answer and recommend not reworking X's answer, because:
It does readers a disservice ...