I discovered a really easy way to deal with font errors. After running Font Book's resolve duplicates followed by its validate option, I noticed many of my fonts had "minor problems", and one font had a "serious problem". My advice is get rid of any fonts that fall into that latter category and fix the one's that have minor issues. To fix a troulesome font (that's not a requried system font), first use Font Book to delete it. Performing this action will likely send your troublesome font to the trash. Remove the font(s) from the trash and drag it(them) on your desktop. Next, use your favorite search engine to perform an online search for "font conversion". Several websites offer free font coversion. Most are fairly simple and intutitive to use. Basically you upload a font and tell the site which kind of font format you want it coverted to. I've found that if I use a free font conversion service to convert a troublesome true type font (.ttf) to an open type (.otf) then install the newly converted .otf font, this will often clear up whatever minor errors the validation process detected the first time. Be sure to recheck the .otf with Font Book's validate option to verify that your converted font now passes the validation process.