Note that this is not the same as: Unable to edit posts with pending edits
Mistagged questions should be retaggable immediately.
Well I just fixed this an slew of similar requests.
If you would like to edit a post that has a pending edit and can not wait, you can click the "improve" button.
This button allows you to "edit the edit", add tags and whatnot.
At the end of the process when you submit the edit, the community user will accept the suggested edit AND your edit will be applied.
We still disable the quick-retag menu, but offer you a way around the issue if you are in a rush.
If you do not have the rep to change the tags without a pending edit do it.
If you can't then well wait until the post is edit and then fix it.