Here is a screen shot of the error message in Font Book.

An age-old problem: the end-user. Not all users keep their systems in the cleanest shape nor do they all use their computer in the same way, so no one answer will fit all. Since you don't have a problem with the font then the font is probably OK. The simplest solution might be to have the users put the "problem" font in ~user/Library/Fonts rather than use Font Book to manage them. Font Book has never been known to be the best of font managers anyway.

The latest versions of FontBook have become particularly sensitive to errors in fonts. Looking at the style of CPL Kirkwood it maybe worth investigating to see if you have some overlapping contours or multiple nodes on the same co-ordinates.

Thank you Malcolm and George. Font Book drives me nuts, it seems to create issues where there is none. I've been trying to research what System Validation Error even means, but haven't found any real answers. Which makes it really difficult for me figure out if it's something I'm on wrong on my end. I hate going back to consumers and telling them that's an issue on their end, so I really want to find a solution.

I will go through and see if there are any overlapping contours / nodes as well, maybe that will help. Thank you so so much!

See also a similar Typophile thread on the subject from a few months back.

Thank you Grzegorz, I've read that thread (a dozen times) but the problem isn't in the kerning feature and I already had hinting enabled (which solved the problem for that person.)

Grzegorz may have a point regarding the hinting of the font especially if you auto-hint on output. If FontLab is creating many, many hints for each glyph it maybe too much for the system, you may need to just insert a single ghost hint at the baseline position for each glyph.

Also for a distressed style of font you should turn off 'Use subroutines to compress outlines...etc'. FontLab may try to create thousands of small contour subroutines that will not be of any benefit and may cause errors.

I am happy to run some diagnostics on your fonts for you if you wish (mwooden at dtptypes dot com).

In a more general note, if your font is more about distressed texture than particular curve control, why not distribute it with TrueType outlines? Both problems Malcolm’s just mentioned are of PostScript nature, so they wouldn’t apply. TrueType outlines are in some measure also more ‘native’ technology to Microsoft and Apple, and both companies deal with it very well. Font Book, for example, would list the table with TrueType outlines and indicate it specifically in case of any problems, whereas the CFF/PostScript table didn’t even show up in the report.