I've come across this bug on both my Macs, and it seems pretty easy to reproduce. Is anybody else having this issue? 1. In Preview, open a screenshot you've taken (or really any image). 2. in the Markup Toolbar, select Shapes. Add a highlight mask to the image (bottom left item in the dropdown). 3. Try to save the image -- you get an error that it "could not be saved". Other annotations (like the arrow and circle I used above) seem to cause no problems, but that shaded mask box triggers this bug. Even if you try to duplicate the image and/or save it in a different format besides PNG, it won't work. Unchecking "alpha" in the save dialog also doesn't help. Seems like a pretty egregious bug for a high-profile MacOS app like Preview.