I keep on getting “save operation failure” after any change on my Xcode Data Model

Question

I started using Core Data for iPhone development. I started out by creating a very simple entity (called Evaluation) with just one string property (called evaluationTopic).

Answer 1

Michael's answer fit my case.
I modified the core data model and starts getting this error.
My solution was remove the app(hold HOME and CROSS the app) and relunch the app. Problem solved!