I was reviewing a bunch of questions, and by mistake flagged an answer as... not a question and downvoted it. I'm very sorry, I didn't notice it was podted as an answer, and now can't reverse my flag nor downvote. What can I do now to make things right?
Unsurprisingly, a moderator has already declined your flag; actually two of these.
As for the downvote, it is likely locked in right now, but if an edit is made to the post you may be able to undo the downvote (by re-clicking the downarrow next to the answer). Now just find a way to have that post edited.... (The other answer you mistakening flagged as "not a question" has received no downvotes.)
Please be more careful when you are flagging posts.
Added to completely answer the question
Once a flag has been raised, there is no way to undo it and call it back. In such cases it is probably advisable to keep an eye on the question/flag to see how we handle it. If we do act on your incorrect flag, you can always flag the post again* and, using the "other" option, explain why your original flag was wrong. (You cannot have multiple active flags on the same post, so you have to wait until something is done with your original flag.) If you can find one of us, e.g., in the main math chat room, you can explain your error there as well (though I guess you will lose your flagging anonymity to regular users in this way).
* Unless you are <10K and we deleted the post. In these cases you'll have to be a bit more creative.
Most of the time, there's probably no need to do anything. If your flag obviously makes no sense, then the moderator handling it will presumably see this, and will decline it. At worst, they might scratch their head wondering what kind of temporary insanity you were suffering from, but I suspect they've seen enough silly flags before — we're all just human, after all, and prone to occasional mistakes.
That said, if you really feel that your flag needs a follow-up message to explain or clarify it, probably the easiest way to send one is just to post it as an ordinary comment on the flagged post. Just start your comment with something like:
"To the moderator handling the flag I just sent: ..."
to avoid needlessly confusing other users. Also, once your flag has been handled by a mod (which you can see in your flag summary, linked from the "helpful flags" number in your user profile), please consider going back and deleting the now-obsolete comment (if it hasn't already been deleted) to keep the site clean and tidy.