Let's say that I want to save a screenshot after a predetermined target name such as Screenshot_2018-03-02_10-11-06.png.

Now, I accidentally click on an existing file inside of that file saving dialogue, and the target name changes towards the existing file.
Pressing Ctrl+Z does unfortunately not revert to the original name.

Hit ESCAPE and save nothing, then do the Save As action again.

I run into this a lot. Weird phenomena with traversing a folder tree. If I go up, then maybe down to a parallel directory, it can "highlight" the first file name in that folder, but it won't put that up in the Save As dialogue. Meanwhile if I move too fast, making an assumption, I'll get the exact same overwrite that you are citing. Yes, it's annoying, and all I've learned to do is to keep my mouse aside from that dialogue so it doesn't do that, and also to hit ESC if I happen to get caught. And then I have to save again ...