Avoid Unwanted/Blank Text Annotations in Preview.app
I might state the obvious. When it's the first (accidental) edit you make in a document, Undo/Command-Z will not only remove the involuntary text box, but will also revert the document state back to 'unchanged'.

After test addition: You may have to click on a 'safe' zone outside of the text box for Undo to function. Thus creating additional 'edits', you have to Undo-away. My experience is, that after two or three undos the 'edited' tag disappears behind the filename in the window title.

Just tried it and had one additional observation:
The only way I found to get the text box to appear was clicking on or near a line in the pdf. That's a feature not a bug: Preview assumes you want to fill out a form.

Hope this helps.

Eerk