I would love to see this feature, although I don't think that misclicking is the real selling point here. As I've already stated in a comment to Calebs answer, the use-case for me is more frequently that other events than my flag being evaluated make the flag irrelevant.
The most frequent example is that the flagged post is edited so that it is now of a quality that would not cause me to flag it in the first place. I know that the reviewers are supposed to see the post as it was when it was flagged, but if it is good now, why not let me retract the flag as events have rendered it a waste of time? HereHere is one such example, where the code was not added (or invisible?) when I flagged it, but there and visible now.
The other, rare, example I've just experienced a few times was after I reviewed a late answer and I ended up placing multiple similar flags - each in essence a superset of the previous as I found out more, and each time I would have liked to retract the previous flag as the newer would provide a more complete picture.
Now the second of the flags have been handled. The other two are still active as I write this and while the third flag is still relevant, the first is factually handled, the flagged answer already deleted and there is nothing more to do on it. But the flag is still active. (That is the Schedule to create AMI weekly from a running instance?
flag below.)
I realise I should probably have waited until I was sure I had 'everything' until placing my flag, but I rarely go deeper once I've placed a flag - especially not from the review queue.
For those interested in the details of the second situation, the answer I reviewed consisted of:
You can go to http://www.SomeCompany.com/TheCoolProduct.html - this tool can do exactly what you need. (Disclaimer: I work at SomeCompany).
The domain of the question was not for me, so I couldn't really tell whether the answer was useful. The user posting didn't provide enough effort for me to automatically accept this as being within these limits, but the user was also new, so a red spam flag might be too harsh.
I flagged it for moderator attention and explained why. Then I got curious, looked some more at the user, and found that all 5 answers he had provided were of the same type. So I flagged one of the other answers and explained the new findings. As mentioned, I would have liked to retract the original flag at this point.
A few minutes later I got more curious and tried searching on SO for SomeCompany. This turned up yet another user with few answers, similar name and similar style answer. So I flagged that as well.
For documentation I've attached a screenshot. As the third flag is still active, I've anonymized the question and the user there as well as my reasons in all three flags.