Disclaimer: Like all good things on Meta, this is an opinion. Feel free to disagree.
Never leave an incorrect answer (even with a note). Consider it this way...
I ask a question where I know the answer but cannot seem to obtain. I get answer saying something along the following:
I did x,y, and z (incredibly detailed steps) but I didn't get the right answer either. I hope this helps the OP with their question.
Then I never receive another answer (because let's face it posts with answers get less traffic).
So basically I get the short end of the stick in this whole thing. Eventually the bots or users will hassle me about "accepting the most helpful answer" and so I'll eventually have to accept the answer. Flagging will do no good as the post has to be intentionally irrelevant (not just incorrect) to be removed by moderators. Therefore, my question is rendered unanswerable as the only way to get an answer at that point would be to bug people about it.
So yeah, don't do that. Make the post contain the right answer. If you want to preserve the old answer refer people to the history of the post. Seriously, just say:
The old answer was incorrect. If you wish to reread it for reference of what not to do click here.
(Put a link to the appropriate history revision in "click here")
But don't refuse to fix the post. Even with a note, you still deprive someone of an answer...