I\'ve seen in the example code supplied by Apple references to how you should handle Core Data errors. I.e:
NSError *error = nil;
if (![context save:&err
No one is going to show you production code because it depends 100% on your application and where the error occurs.
Personally, I put an assert statement in there because 99.9% of the time this error is going to occur in development and when you fix it there it is highly unlikely you will see it in production.
After the assert I would present an alert to the user, let them know an unrecoverable error occurred and that the application is going to exit. You can also put a blurb in there asking them to contact the developer so that you can hopefully track this done.
After that I would leave the abort() in there as it will "crash" the app and generate a stack trace that you can hopefully use later to track down the issue.