In the Suggested Edits review queue, if you select Improve and then immediately press save (without making any improvements), this error message appears:
You seem to be improving a post however, you made no changes.
There is a punctuation error. Also, "You seem to be" is unnecessarily verbose, since the software knows for sure that you are improving a post. It should instead be something like
Improving a post requires making changes.
When improving a post, you must make changes.